Chromosomes and Phenotype Presentation PDF

KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits. ⮚ Recall: Sex chromosomes determine an organism’s sex. Humans can be X X (female) or X Y (male) Autosomes are all of the other chromosomes, and they do not play a direct role in sex determina...

KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits. ⮚ Recall: Sex chromosomes determine an organism’s sex. Humans can be X X (female) or X Y (male) Autosomes are all of the other chromosomes, and they do not play a direct role in sex determination. Autosom es Sex  Two copies of each autosomal gene affect phenotype. Mendel studied autosomal gene traits, like hair texture. Most of the traits are autosomal traits that are carried on autosomes (22 pairs in human) and the genotype always represented by English letters like the next example (eye color) ⮚Disorders Caused by Recessive Alleles on autosomes: Two copies of the recessive allele must be present for a person to have the disorder. Someone who is heterozygous for the alleles will not have the disease but is a carrier EXAMPLE: (autosomal recessive) Cystic fibrosis Autosomal recessive disease C: healthy c: Affected Recessive disorders are mostly more common than dominant disorders.  ⮚ Disorders Caused by Dominant Alleles on autosomes: EXAMPLE: (autosomal dominant) – Huntington’s disease (damage the nervous system) – H: affected – h: healthy Since the disorder is dominant, thus whenever the affected allele is present, the person will be affected.  Males and females can differ in sex-linked traits. Genes on sex chromosomes are called sex-linked genes. – Y chromosome genes in mammals are responsible for male characteristics. – X chromosome genes in mammals affect many traits. ⮚ Expression of Sex-Linked Genes In males, there are no second copies of sex-linked genes to mask the effects of another allele. This means that even if all of the alleles of sex-linked genes in a male are recessive, they will be expressed.​ Recessive disease Affetc ed Males cannot be carriers for a sex-linked disease. (either healthy or affected) ⮚Sex-linked disease example: One example of a sex-linked trait is color blindness, which is primarily seen in males. Examples of sex-linked traits: ‒ Haemophilia & color blindness (X-linked recessive) ‒ Rett Syndrome (x-linked dominant) Rett Syndrome (rare): affect brain development XH: Healthy XR: XC : Healthy Affetced c  Male mammals have an XY genotype. – All of a male’s sex- linked genes are expressed. – Males have no second copies of sex-linked genes.  Female mammals have an XX genotype. – Expression of sex-linked genes is similar to autosomal genes in females. – X chromosome inactivation randomly “turns off” one X chromosome. ⮚ Codominance (multiple allele trait) : Both alleles of a gene are expressed completely—neither allele is dominant nor recessive Blood group A B AB O ⮚ Practice 1 What will be the predicted F1 offspring blood groups for a male with IA i blood group married to a female with IB i blood group. 25% blood group AB IA IB 25% blood group A IA i IB i 25% blood group B i IB 25% blood group O ii IA IA IB IA i i i IB ii  Many genes may interact to produce one trait ⮚ Polygenic Traits Traits produced by two or more genes Example: Human skin color, eye color: the green allele is dominant to blue alleles, but it is recessive to all brown alleles ⮚ Epistasis at least five different genes interact to produce the phenotype But one gene of all can overshadow all other genes. This gene is called Epistatic gene (a gene that interfere with the expression of all other genes for a specific trait) Example: mouse fur, Albinism Albinism: one gene blocks the production of pigments called the epistatic gene  ⮚ The environment interacts with genotype: phenotype is usually a mixture of genes and environment. Example: Female turtles make nests on beaches and bury their eggs in the sand. Eggs that mature in warmer temperatures develop into female turtles. Eggs that mature in cooler temperatures develop into male turtles. ⮚ Practice 2: Rett syndrome is an x-linked dominant syndrome (Rett syndrome is a rare genetic neurological and developmental disorder that affects the way the brain develops). What will be the possible F1 generation if a normal male married a heterozygous female with Rett syndrome. Step 1: clarify the letters to be used Step 2 : draw the Punnett square Step 3: find the phenotypes and genotypes ⮚ Practice 2 answer: S: affected s: Normal XS Xs Phenotypes: 50% of the offspring will be affected Xs XSXs XsXs Y XSY XsY Genotype: 25% female affected 25% female normal 25% male affected 25% male normal (having Rett syndrome) ⮚ Practice 3: Hypertrichosis of the ears is thought as a Y-linked disorder. What will be the offspring F1 phenotype for a male with hypertrichosis married a normal female. Step 1: Step 2: Step 3: Answer: X X h h H: affected X XX h XhX h: normal h h h Y XhY XhY All females will be normal (no affected or carrier female H H H must be present) All males will be affected  ⮚ Practice 4: A curly haired man was married to a straight haired woman and the F1 generation showed 100 % wavy haired offspring. What is the type of inheritance that codes hair texture in this example. Answer: Since the F1 generation shows a completely new phenotype (wavy), this inheritance is an incomplete dominance type. ⮚ PRACTICE 5: Hemophilia is an X-linked recessive inherited disorder in which the blood can’t clot properly leading to spontaneous bleeding. If a normal male (XHY) was married to a heterozygous female that carry a gene responsible for hemophilia (XHXh). What will be the percentage of having hemophilic baby? Using the Punnett square, 25% of the offspring might be hemophilic. What will be the percentage of having hemophilic males? Females? 50 % of the males might be affected, 0% of the females might be affected (no females will be affected) In which gender hemophilia is more likely to appear? why? X X Males, because it is linked to the X-chromosome H h where males have only one copy of it, so if this X- H X XHX H XHX h chromosome was carrying the disorder, the male Y XH Xh will be affected. Y Y Practice 6: Color blindness is an X-linked recessive disorder. 1. If a color blind male was mate with a heterozygous normal female, what would be the Predicted F1 generation? 2. What is the probability of affected offspring over the normal ones? X Color blind carrier c allele X Normal X X C 1. allele F1: 50 % of the females will be heterozygous X X C C c Xc normal c XcC Xcc 50% of the females will be affected (color Y X X blindness) Y Y 50% of the males will be affected 50% of the males will be color blinded 2. 50% of the F1 generation will be affected  Recall: Females have an XX chromosomes, so can be carriers of an x-linked disorder Males have XY chromosomes (only one copy of each chromosome) males cannot be carriers for x-linked disorder (either normal or affected) Males are more likely to be affected by a sex- linked disorder. For autosomal disorders, male an females can be homozygous or heterozygous. (genes pass equally to offspring)  Pedigree: A pedigree is a chart that can help trace the phenotypes and genotypes in a family to determine whether people carry recessive alleles. Autosomal: because both genders are affected  ⮚ Autosomal dominant: ✔ males and females are equally likely to be affected ✔ All generation have affected offspring ✔ both males and females transmit the disease ✔ transmission from father to son ⮚Autosomal Recessive: ✔ Not all generations have an affected offspring ✔ If both parents express the trait, then all their offspring should also express the trait  Widow’s peak (autosomal dominant example) A widow's peak is a V- shaped point in the hairline in the center of the forehead W: dominant (widow’s peak allele) w: Recessive no widow’s peak In an autosomal trait, both males and females are likely to be inherited that trait equally.  Practice 9: Autosomal dominant trait: Find the genotype of the following offspring: II) 2, 3, 5 III) 1 IV) 1,2,3 and 4  Practice 9 Answer: II) 2)W W3) W 4) w 5) III) 1)ww w w w IV) 1)ww w 2) W 3) w 4) w w w w  Practice 11: Is the trait autosomal or sex-linked. Dominant or recessive This trait is an autosomal recessive it is autosomal because the It is not sex-linked mother is affected while some dominant boys in F1 are normal. (they must (otherwise all F1 inherit an affected x from the girls will be affected mother, not the case) for affected father) Autosomal recessive: Normal parents have affected offspring Practice 12 a. aa x Aa 🡪 50% normal, 50 % affected b. Aa x aa 🡪 50% normal, 50% affected c. Xa Y x XAXa 🡪 50% normal, 50% affected d. XA Y male will have all daughters affected , which is not the case Practice Recessive; since 13 XA XA we have normal Y Xa parents with affected boy. XA X Xa XA A Y Xa XA XA Xa Y Xa Y XA Xa Sex-linked: More males are affected than females  ⮚ Practice 13: 1. Is the trait in the given pedigree autosomal or sex- linked? Explain your answer. Autosomal, boys and girls are equally affected. 2. Is the trait dominant or recessive? explain. - Recessive, affected parents have 100% affected offspring. (II4,5) - Parents II) 1 and 2: affected parent have 100% normal offspring. 3. Find the genotype of the following offspring: I) 2 ______ a II) 1 ______and aA 2 ______ a a be heterozygous Aa for 100% not affected offspring) A male cannot 1 AA(since 4%  Practice 7:  Practice 8: Genes A,B, C and D are linked to the same chromosome. The recombination frequencies (RF) are as follows: Relationship Draw the genetic map for the given set of genes. RF What is the order of genes in the chromosome? B–D 14 % C–D 12 % Order: BCAD or A–D 6% DACB B–C 2% 2 % A–B 8% 6 B C A % D 6 % 14 %