Development and Inheritance 2020 PDF

Development and Inheritance I Patel 2020 Describe the various stages of development Fertilisation (conception) Prenatal development Embryological development (1st 2 months) Foetal development (9th week till birth) Postnatal development and Maturity...

Development and Inheritance I Patel 2020 Describe the various stages of development Fertilisation (conception) Prenatal development Embryological development (1st 2 months) Foetal development (9th week till birth) Postnatal development and Maturity 2 An Introduction to Development and Inheritance 3  An Introduction to Development and Inheritance Development Gradual modification of anatomical structures and physiological characteristics from fertilization to maturity Differentiation Creation of different types of cells required in development Occurs through selective changes in genetic activity As development proceeds, some genes are turned off, others are turned on Fertilization Also called conception When development begins 4  Development… Embryonic Development Occurs during first two months after fertilization Study of these events is called embryology Fetal Development Begins at start of ninth week Continues until birth Prenatal Development Embryonic and fetal development stages Postnatal Development Commences at birth Continues to maturity, the state of full development or completed growth Inheritance Transfer of genetic material from generation to generation 5  Fertilization Fusion of two haploid gametes, each containing 23 chromosomes Produces zygote containing 46 chromosomes Spermatozoon Delivers paternal chromosomes to fertilization site Travels relatively large distance Is small, efficient, and highly streamlined 6 7 Re-Cap 8 9  Female Gamete Provides: Cellular organelles Inclusions Nourishment Genetic programming necessary to support development of embryo for a week Much Larger than sperm Suspended in metaphase II 10 100 10 thousand 200 million 300 million 11  Capacitation Must occur before spermatozoa can fertilize secondary oocyte Contact with secretions of seminal glands Exposure to conditions in female reproductive tract Q: What prevents premature capacitation? Q2: From where is this substance secreted? Fertilization Occurs in uterine tube within a day after ovulation Secondary oocyte travels a few centimeters Spermatozoa must cover distance between vagina and ampulla 12  Oocyte at ovulation Secondary oocyte suspended in metaphase of Meiosis II Metabolic functions suspended Surrounded by corona radiata – which protects the secondary oocyte If not fertilised it will disintegrate 13  Oocyte at Ovulation Ovulation releases a secondary oocyte and the first polar body; Fertilization both are surrounded by the corona radiata. The oocyte is suspended in metaphase of meiosis II. Corona First polar radiata body Zona pellucida 14  Acrosomes Release hyaluronidase and acrosin Penetrate corona radiata, zona pellucida, toward oocyte surface Hyaluronidase Enzyme breaks down bonds between adjacent follicle cells Allows spermatozoon to reach oocyte Acrosin Is a proteolytic enzyme Is required to reach oocyte Digests a pathway to the surface of the oocyte 15 1 Fertilization and Oocyte Activation Acrosomal enzymes from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the oocyte membrane, and membrane fusion occurs, triggering oocyte activation and the completion of meiosis. Fertilizing Second polar spermatozoon body 16  Oocyte Activation Involves a series of changes in metabolism of the oocyte Contact and fusion of cell membranes of sperm and oocyte Follows fertilization Oocyte completes meiosis II, becomes mature ovum Fusion of sperm and oocyte membranes triggers oocyte membrane depolarization – due to increase in membrane sodium permeability Sodium influx triggers calcium release from smooth ER leading to cortical reaction 17 i) Cortical reaction - enzymes (ZIPS) released to harden zona pellucida and inactivate sperm receptors (prevents polyspermy) ii) Completion of meiosis II + 2nd polar body  Ovum iii) Enzyme activation and Increased metabolic rate (activation of mRNA) 18  Female Pronucleus Nuclear material remaining in ovum after oocyte activation Male Pronucleus Swollen nucleus of spermatozoon Migrates to center of cell 19 2 Pronucleus Formation Begins The sperm is absorbed into the cytoplasm, and the female pronucleus develops. Nucleus of Female fertilizing pronucleus spermatozoon 20 21  Amphimixis Fusion of female pronucleus and male pronucleus Moment of conception Cell becomes a zygote with 46 chromosomes Fertilization is complete 22 4 Amphimixis Occurs and Cleavage Begins Metaphase of first cleavage division 23  Cleavage Series of cell divisions Produces daughter cells Differentiation Involves changes in genetic activity of some cells but not others Release of chemical substances (RNA, proteins etc) can affect differentiation of other embryonic cells (Induction) 24 Figure 29-1b Fertilization (Part 6 of 6). 5 Cleavage Begins The first cleavage division nears completion about 30 hours after fertilization. Blastomeres 25  List the 3 stages of prenatal development Describe the events of each of the 3 stages of prenatal development 26  Terminology Q: If all cells of the embryo are derived from cell divisions of the zygote, how do cytoplasmic differences originate? What effect do these difference have on the developing fetus? 27  Terminology Induction Cells release chemical substances that affect differentiation of other embryonic cells Can control highly complex processes Gestation Time spent in prenatal development Consists of three integrated trimesters, each three months long 28  Overview 1. First Trimester Period of embryonic and early fetal development Rudiments of all major organ systems appear 2. Second Trimester Development of organs and organ systems Body shape and proportions change 3. Third Trimester Rapid fetal growth and deposition of adipose tissue Most major organ systems are fully functional 29  Describe the four general processes that occur during the first trimester 30  At conception: fertilized ovum = single cell 0,135mm and 150ug After 1st trimester: Fetus = 75mm and 14g First Trimester Includes four major stages 1. Cleavage 2. Implantation 3. Placentation 4. Embryogenesis 31  Cleavage Sequence of cell divisions begins immediately after fertilization Zygote becomes a pre-embryo, which develops into multicellular blastocyst Ends when blastocyst contacts uterine wall Implantation Begins with attachment of blastocyst to endometrium of uterus Sets stage for formation of vital embryonic structures 32  Placentation Occurs as blood vessels form around periphery of blastocyst and placenta develops Placenta is important for maternal and fetal exchange of material Embryogenesis Formation of viable embryo Establishes foundations for all major organ systems 33  The First Trimester Most dangerous period in prenatal life 40 percent of conceptions produce embryos that survive past first trimester 34  Cleavage and Blastocyst Formation Blastomeres Identical cells produced by cleavage divisions Morula Stage after three days of cleavage Pre-embryo is solid ball of cells resembling mulberry Reaches uterus on day 4 35  Blastomeres Polar bodies 4-cell stage 2-cell stage DAY 1 DAY 2 First cleavage division DAY 0: Fertilization 36  Blastocyst Formed by blastomeres Hollow ball with an inner cavity Known as blastocoele Trophoblast Outer layer of cells separate outside world from blastocoele Cells responsible for providing nutrients to developing embryo 37  Inner cell mass Clustered at end of blastocyst Exposed to blastocoele Insulated from contact with outside environment by trophoblast Will later form embryo 38  Zona pellucida Early morula DAY 3 Advanced DAY 4 morula Hatching Inner cell mass DAY 6 Blastocoele Days 7–10: Implantation in Trophoblast uterine wall Blastocyst (see Figure 29–3) 39  Implantation Occurs seven days after fertilization Blastocyst adheres to uterine lining Trophoblast cells divide rapidly, creating several layers Cellular trophoblast Cells closest to interior of blastocyst Syncytial trophoblast Outer layer Erodes path through uterine epithelium by secreting hyaluronidase 40 DAY 6 FUNCTIONAL ZONE UTERINE OF ENDOMETRIUM CAVITY Uterine glands Blastocyst DAY 7 Trophoblast Blastocoele Inner cell mass 41  Ectopic Pregnancy Implantation occurs outside uterus Does not produce viable embryo Can be life threatening 42  Formation of the Amniotic Cavity Lacunae Trophoblastic channels carrying maternal blood Villi extend away from trophoblast into endometrium Increase in size and complexity until day 21 Amniotic Cavity A fluid-filled chamber Inner cell mass is organized into an oval sheet two layers thick Superficial layer faces amniotic cavity Deeper layer is exposed to fluid contents of blastocoele 43 DAY 8 Endometrial capillary Cellular Syncytial trophoblast trophoblast DAY 9 Developing villi Amniotic cavity Lacuna 44  Explain how the three germ layers participate in the formation of extra-embryonic membranes 45  Gastrulation and Germ Layer Formation Formation of third layer of cells Cells in specific areas of surface move toward central line Known as primitive streak 46 Amniotic fluid and yolk sac formation & Gastrulation 47  Primitive Streak Migrating cells leave surface and move between two layers Creates three distinct embryonic layers, or germ layers 1. Ectoderm: consists of the superficial cells that did not migrate into interior of inner cell mass 2. Endoderm: consists of cells that face blastocoele 3. Mesoderm: consists of poorly organized layer of migrating cells between ectoderm and endoderm 48  Embryonic Disc Oval, three-layered sheet Produced by gastrulation Will form body of embryo Rest of blastocyst will be involved in forming extraembryonic membranes 49  Endocrine system: Pituitary gland and adrenal medullae Respiratory system: Mucous epithelium of nasal passageways Digestive system: Mucous epithelium of mouth and anus, salivary glands 50  Lymphatic system: All components Urinary system: The kidneys, including the nephrons and the initial portions of the collecting system Reproductive system: The gonads and the adjacent portions of the duct systems Miscellaneous: The lining of the body cavities (pleural, pericardial, and peritoneal) and the connective tissues that support all organ systems 51  Formation of the Extraembryonic Membranes Support embryonic and fetal development Germ layers are involved in the formation of FOUR extra-embryonic membranes: Yolk sac (endo- and mesoderm) Amnion (ecto- and mesoderm) Allantois (endo- and mesoderm) Chorion (meso- and trophoblast) 52  The Yolk Sac Begins as layer of cells spread out around outer edges of blastocoele to form complete pouch Important site of blood cell formation The Amnion Combination of mesoderm and ectoderm Ectodermal layer enlarges and cells spread over inner surface of amniotic cavity Mesodermal cells create outer layer Continues to enlarge through development Amniotic fluid Surrounds and cushions developing embryo or fetus 53  The Allantois Sac of endoderm and mesoderm Base later gives rise to urinary bladder The Chorion Combination of mesoderm and trophoblast Blood vessels develop within mesoderm Rapid-transit system for nutrients that links embryo with trophoblast First step in creation of functional placenta Chorionic Villi In contact with maternal tissues Create intricate network within endometrium carrying maternal blood 56  Placentation Chorion continues to enlarge By week 4 the embryo, amnion and yolk sac are suspended in a fluid filled chamber Fetus moves further away from placenta Connected by umbillical chord – allantois, placental blood vessels and yolk stalk 57 S Eagleton 2010 Figure 29-6a Views of Placental Structures (Part 3 of 3). Chorionic villi Umbilical vein Umbilical arteries Area filled with maternal blood Amnion Maternal blood vessels Trophoblast (cellular and syncytial layers) a A view of the uterus after the fetus has been removed and the umbilical cord cut. Arrows in the enlarged view indicate the direction of blood flow. Blood flows into the placenta through ruptured maternal arteries and then flows around chorionic villi, which contain fetal blood vessels. 61  Embryogenesis Body of embryo begins to separate from embryonic disc Body of embryo and internal organs start to form Folding, differential growth of embryonic disc produces bulge that projects into amniotic cavity Projections are head fold and tail fold Organogenesis Process of organ formation 62 The First 12 Weeks of Development Future head of embryo Thickened neural plate (will form brain) Axis of future spinal cord Somites Neural folds Cut wall of amniotic cavity Future tail of embryo Week 2. An SEM of the superior surface of a monkey embryo at 2 weeks of development. A human embryo at this stage would look essentially the same.  Medulla The First 12 Weeks of Development oblongata Ear Pharyngeal Forebrain arches Eye Heart Somites Body stalk Arm bud Tail Leg bud Week 4. Fiberoptic view of human development at week 4.  The First 12 Weeks of Development Chorionic villi Amnion Umbilical cord Placenta Week 8. Fiberoptic view of human development at week 8.  The First 12 Weeks of Development Amnion Umbilical cord Week 12. Fiberoptic view of human development at week 12. Development and Inheritance - 2 I Patel 2020  Describe the role of the placenta as an endocrine organ 68  The Endocrine Placenta All hormones synthesized by syncytial trophoblast, released into maternal bloodstream 1) Human chorionic gonadotropin (hCG) 2) Human placental lactogen (hPL) 3) Placental prolactin 4) Relaxin 5) Progesterone 6) Estrogens 69 1. Human Chorionic Gonadotropin (hCG) Appears in maternal bloodstream soon after implantation Provides reliable indication of pregnancy Pregnancy ends if absent Maintains integrity of corpus luteum 3-4 months Promotes progesterone to maintain endometrial lining 70 2. Human Placental Lactogen (hPL) Human chorionic somatomammotropin (hCS) Prepares mammary glands for milk production Synergistic with growth hormone at other tissues Ensures adequate glucose and protein is available for the fetus S Eagleton 2010 71 3. Placental Prolactin Helps convert mammary glands to active status 4. Relaxin A peptide hormone secreted by placenta and corpus luteum during pregnancy 1. Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery 2. Causes dilation of cervix 3. Suppresses release of oxytocin by hypothalamus and delays labor contractions 5. Progesterone and Estrogen Placenta produces sufficient amounts after first trimester of pregnancy End of third trimester estrogen levels increase stimulating labor and delivery 73  Second Trimester Fetus grows faster than surrounding placenta Third Trimester Most of the organ systems become ready Growth rate starts to slow Largest weight gain Fetus and enlarged uterus displace many of mother’s abdominal organs 74  The Second and Third Trimesters Head 6 month fetus (ultrasound) 4 month fetus (fiber-optic endoscope) S Eagleton 2010  Describe the interplay between the maternal organ systems and the developing fetus 76  Pregnancy and Maternal Systems Developing fetus is totally dependent on maternal organ systems for nourishment, respiration, and waste removal Maternal adaptations include increases in: Respiratory rate and tidal volume Blood volume Nutrient and vitamin intake Glomerular filtration rate Size of uterus and mammary glands 77 Growth of the Uterus and Fetus 78  Explain the hormonal changes that play a role during the onset of labour. Describe the role of mechanical changes in the initiation of labour. 79  Structural and Functional Changes in the Uterus False labor Occasional spasms in uterine musculature Contractions not regular or persistent True labor Results from biochemical and mechanical factors Continues due to positive feedback Labor contractions Begin in myometrium 80  Progesterone Released by placenta Has inhibitory effect on uterine smooth muscle Prevents extensive, powerful contractions Opposition to Progesterone Three major factors 1. Rising estrogen levels 2. Rising oxytocin levels 3. Prostaglandin production 81  Labor - Hormonal control Ratio of estrogen to progesterone Progesterone inhibits uterine contractions Estrogen increases uterine contractility – by increasing gap junctions between smooth muscle cells Ratio changes towards end of pregnancy Effects on oxytocin release Increased # of receptors for oxytocin Increase rate of release of oxytocin Without it prolonged labor Neuroendocrine reflex due to stretching of cervix 82  Effects of fetal hormones Oxytocin Prostaglandins S Eagleton 2010 83  Labor - Mechanical factors Stretch of uterine muscles Stretching initiates contractions Twins normally born 19 days earlier than single child – related to mechanical stretch Stretch or irritation of the cervix Head of baby stretches cervix and irritates it Mechanism of feedback to uterus unclear – either reflex or myogenic transmission 84 Initiation of Labor  Parturition Is forcible expulsion of fetus Contractions Begin near top of uterus, sweep in wave toward cervix Strong, occur at regular intervals, increase in force and frequency Change position of fetus, move it toward cervical canal 87  Briefly describe the stages of labour. 88  Stages of Labor Labour has traditionally been divided into three stages: 1. Dilation stage 2. Expulsion stage 3. Placental stage 89 1. Dilation Stage Begins with onset of true labor Cervix dilates Fetus begins to shift toward cervical canal Highly variable in length, but typically lasts over eight hours Frequency of contractions steadily increases Amniochorionic membrane ruptures (water breaks) 90 2. Expulsion Stage Begins as cervix completes dilation (10cm) Contractions reach maximum intensity Continues until fetus has emerged from vagina Typically less than two hours Delivery Arrival of newborn infant into outside world 91 3. Placental Stage Muscle tension builds in walls of partially empty uterus Tears connections between endometrium and placenta Ends within an hour of delivery with ejection of placenta, or afterbirth Accompanied by a loss of blood 92 The Stages of Labor 93  Immature Delivery Refers to fetuses born at 25–27 weeks of gestation Most die despite intensive neonatal care Survivors have high risk of developmental abnormalities Premature Delivery Refers to birth at 28–36 weeks Newborns have a good chance of surviving and developing normally 94  Multiple Births Dizygotic twins Also called “fraternal” twins Develop when two separate oocytes were ovulated and subsequently fertilized by two separate spermatozoa Genetic makeup not identical 70% of twins Monozygotic twins Also called “identical” twins Result either from: Separation of blastomeres early in cleavage Splitting of inner cell mass before gastrulation Genetic makeup is identical because both formed from same pair of gametes 95  Briefly discuss the following stages: 1. The neonatal period 2. Infancy and childhood 3. Adolescence and maturity 4. Senescence 96  Duration of Life Stages Neonatal Period: extends from birth to 1 month Infancy: 1 month to 2 years of age Childhood: 2 years until adolescence Adolescence: period of sexual and physical maturation Senescence: process of aging that begins at end of development (maturity) 97  The Neonatal Period, Infancy, and Childhood Two major events occur 1. Organ systems become fully operational 2. Individual grows rapidly and body proportions change significantly Pediatrics Medical specialty focusing on postnatal development from infancy to adolescence 98  The Neonatal Period Transition from fetus to neonate Neonate Newborn Systems begin functioning independently Respiratory Circulatory Digestive Urinary Temperature control 99  Feeding Colostrum more proteins and less fat than milk antibodies Breast Milk water, proteins, amino acids, lipids, sugar, salts lysozomes 750 calories per litre 100  Infancy and Childhood Growth occurs under direction of circulating hormones Growth hormone Adrenal steroids Thyroid hormones Growth does not occur uniformly Body proportions gradually change 101  Growth and Changes in Body Form and Proportion Prenatal Development Embryological Development Fetal Development 4 weeks 8 weeks 16 weeks 102  Postnatal Development Neonatal Infancy Childhood Adolescence Maturity 1 month 2 years Puberty 18 years (between 9–14 years)  Adolescence and Maturity Puberty is a period of sexual maturation and marks the beginning of adolescence Generally starts at age 12 in boys, age 11 in girls Three major hormonal events interact 1. Hypothalamus increases production of GnRH 2. Circulating levels of FSH and LH rise rapidly 3. Ovarian or testicular cells become more sensitive to FSH and LH Hormonal changes produce sex-specific differences in structure and function of many systems 104  Adolescence Begins at puberty Continues until growth is completed Maturity (Senescence) Aging Reduces functional capabilities of individual Affects homeostatic mechanisms Sex hormone levels decline at menopause or male climacteric 105 S Eagleton 2010 Development and Inheritance - 3 I Patel 2020  Explain the role of the Y chromosome ensuring that the bipotential embryo develops into a male Explain inheritance using punnet squares and pedigrees Explain sources of individual variation and sex-linked inheritance 108  Introduction Nucleated Somatic Cells Carry copies of original 46 chromosomes present in zygote Genotype Chromosomes and their component genes Contain unique instructions that determine anatomical and physiological characteristics Derived from genotypes of parents Phenotype Physical expression of genotype Anatomical and physiological characteristics 109  Patterns of Inheritance Homologous chromosomes Two members of each pair of chromosomes 23 pairs carried in every somatic cell At amphimixis, one member of each pair is contributed by spermatozoon, other by ovum Autosomal chromosomes 22 pairs of homologous chromosomes Most affect somatic characteristics Each chromosome in pair has same structure and carries genes that affect same traits 110  Sex chromosomes Last pair of chromosomes Determine whether individual is genetically male or female Locus Gene’s position on chromosome Alleles are various forms of given gene Alternate forms determine precise effect of gene on phenotype 111 A Human Karyotype  Patterns of Inheritance Homozygous Both homologous chromosomes carry same allele of particular gene 80% of individuals genome consists of homozygous alleles Heterozygous Homologous chromosomes carry different allele of particular gene Resulting phenotype depends on nature of interaction between alleles 114  Interactions between Alleles Simple inheritance Phenotype determined by interactions between single pair of alleles Strict dominance Dominant allele expressed in phenotype, regardless of conflicting instructions carried by other allele Recessive allele Expressed in phenotype only if same allele is present on both chromosomes of homologous pair Albinism Incomplete dominance Heterozygous alleles produce unique phenotype Sickling gene Codominance Exhibits both dominant and recessive phenotypes for traits 115  Penetrance Percentage of people with a specific genotype that show the “expected” phenotype Expressivity The extent to which a particular allele is expressed when it is present Teratogens Factors that result in abnormal developement 116 117  Predicting SIMPLE Inheritance (Punnet Square) Dominant alleles are capitalised Recessive alleles are lowercase AA - homozygous dominant Aa – heterozygous aa – homozygous recessive Relates to autosomal characteristics 118  Red Genotype: 4Aa; 0AA; 0aa Phenotype: 4 Red; 0 Blue Blue S Eagleton 2010 S Eagleton 2010  Genetic Pedigree Useful for determining inheritance of genes in a population Series of symbols are used Phenotypes are assessed and genotypes are thus determined 126 Common Pedigree Symbols Male Marriage Female Sex Unknown Consanguineous Marriage Affected Female No Known Pregnancy Female Carrier of Sex- linked Recessive Unmarried Female Carrier (Heterozygous) Divorce and Remarrie Dead 127 Common Pedigree Symbols Pregnancy in Dizygotic Progress (nonidentical, fraternal) twins 3 Three Males Five Individuals (both Monozygotic 5 sexes) (identical) twins Spontaneous Abortion Proband, propositus, or index Adopted Child case 128  Affected individuals have at least one affected parent The phenotype generally appears every generation Two unaffected parents only have unaffected offspring  Unaffected parents can have affected offspring Affected progeny are both male and female 131 S Eagleton 2010  The parents of a child with severe sensorineural hearing loss are referred to the genetics clinic. They indicate that they have had three children: a son, a daughter and then another son, and that it is their daughter who was found at the age of 9 months to have severe hearing loss. The father of these children has two sisters both of whom have two sons. His parents are both well. No-one in his family has ever had a child with hearing loss before. The mother of the three children has a brother who has one son and one daughter. Her parents are also well and once again there is no known history of severe hearing loss in childhood. However, her maternal grandfather developed moderate hearing loss in his sixties. On specific questioning the mother recalls that her maternal grandmother was the sister of her husband’s paternal grandmother. Now draw up the family tree. What is the most likely genetic explanation for the daughter’s severe hearing loss? 133 S Eagleton 2010  Margaret has just learned that she has adult polycystic kidney disease. Her mother also has the disease, as did her maternal grandfather and his younger brother (both of whom are now dead). As far as Margaret knows, no one else in her extended family has the disease, although she had a sister, Allison, who died in a car accident when she was 16 and might have showed symptoms if she had lived long enough. Margaret is 42 years old and has three children with her husband, Art. Anna is 20, Lydia is 18, and Tom is 15. Her husband’s name is Art. Draw a pedigree for this family, using the proper symbols. (Hint: Do not use names.) S Eagleton 2010 135 S Eagleton 2010  Could this disease be autosomal recessive? If so, is this mode of inheritance likely? Explain. Yes, but it's unlikely. In order for the inheritance to be autosomal recessive, I-1 and II-1 would both have mated into the family as carriers. Unless there is inbreeding, the chance of this is remote. S Eagleton 2010 137  Could this disease be autosomal dominant? If so, what is the chance that Lydia will get the disease when she reaches middle-age? Yes. The disease shows the classic pattern for autosomal dominant inheritance. It is seen in every generation and no one with the disease has to mate into the family. Lydia's chance of inheriting the disease allele from her mother is 0.5 because her mother had a normal father and is therefore heterozygous. If Lydia received the allele, she will eventually get the disease (if she lives long enough). S Eagleton 2010 138  Polygenic Inheritance Some phenotypic traits are determined by the interaction between several genes Resulting phenotype depends on how alleles interact Suppression One gene suppresses the other, which then has no effect on phenotype Complementary Gene Action Dominant and recessive alleles on two different genes interact 139  Sex-Linked Inheritance Sex Chromosomes X Chromosome Considerably larger than Y Has more genes than does Y chromosome Carried by all oocytes Y Chromosome Includes dominant alleles specifying that the individual will be male Not present in females 140  Sperm Carry either X or Y chromosome Because males have one of each, can pass along either X-Linked Genes that affect somatic structures Carried by X chromosome Inheritance does not follow pattern of alleles on autosomal chromosomes 141 Inheritance of an X-Linked Trait A woman—who has two X chromosomes—can be either homozygous dominant (XCXC) or heterozygous (XCXc) and still have normal color vision. She will be unable to distinguish reds from greens only if she carries two recessive alleles, XcXc. A man has only one X chomosome, so Normal female Normal female whichever allele that (carrier) chromosome carries determines whether he has normal color vision or is red–green Normal male Color blind color blind. male  Some Facts more Y linked traits only appear in males X linked recessive traits are frequent in males haemophilia colour blindness duchenne muscular dystophy 143  DEFECTIVE NORMAL Males ONLY HAVE ONE X They either Or They are have the normal disorder NORMAL DEFECTIVE FEMALES HAVE TWO X CHROMOSOMES Females have Females need two one normal defective recessive gene that works. alleles to show the disorder  Sex determination SRY gene Found on the Y chromosome Gonads are initially undifferentiated (bipotential) Male specific transcription factors allow male differentiation Testis will then develop (8th week) If not present, gonads will develop into ovaries (12th week) 148