Behavioral Biology Lecture 13: Genetic Inheritance PDF

Behavioural Biology Lecture 13: Genetic Inheritance  Read: Chapter 20 Today’s Lecture Gregor Mendel (Father of genetics) How can we predict which traits our offspring will inherit? Pedigrees Degrees of dominance between alleles Multiple Alleles X-linked Chara...

Behavioural Biology Lecture 13: Genetic Inheritance  Read: Chapter 20 Today’s Lecture Gregor Mendel (Father of genetics) How can we predict which traits our offspring will inherit? Pedigrees Degrees of dominance between alleles Multiple Alleles X-linked Characters Environmental effects on genetic expression Mental health and genetics Gregor Mendel (1822 – 1884) An Austrian monk, considered the father of modern genetics Discovered how traits are passed on from one generation to the next. How did Mendel do this without modern technology? Mendel experimented with pea plants. He chose plants with certain traits and cross bred them with plants with other traits. The phenotypes of the offspring were quantified and analyzed. E.g. Purple flowers X white flowers. Mendel’s Experiment What we know now: PP X pp Pp Pp X Pp PP, Pp, and pp Ratio of 3:1 Offspring inherit one allele from each parent. Sperm and egg cells contain only one copy of each chromosome pair and therefore have only one allele for each gene. Example: The dad’s genotype for gene A is Aa, 50% of his sperm cells will have the A allele and 50% will have the a allele. How can we know the chances of inheriting certain traits from our Punnett Square Determines the probability of an offspring having a particular genotype. The Punnett square is a simple grid device that shows the expected frequencies of genotypes in offspring. The rows represent the possible gametes of one parent The columns represent the possible gametes of another parent The boxes represent the possible genotypes of offspring. Recessive Genetic Disorders Albinism To have a recessive disorder (express it in their phenotype) an individual must be homozygous recessive. The recessive allele for the Cystic Fibrosis gene is faulty in some way: Makes required protein incorrectly or not at all. If an individual is heterozygous for the disease they are referred to as Carriers, but the disorder is Sickle-cell Disease  Recessive Genetic Disorders Example: Albinism A = allele for normal pigmentation a = faulty allele causing albinism Possible Genotypes: AA Aa (carrier) aa Phenotypes: Normal pigmentation Normal pigmentation Albinism Dominant Genetic Disorders Disorder expressed if the individual has both dominant alleles or is heterozygous. Examples: Polydactyly Achondroplasia (form of dwarfism) Achondroplasia Polydactyly (extra fingers or toes) Huntington’s Chorea Huntington’s Chorea (degenerative disease) Late onset Dominant Genetic Disorders Example: Genetic Dwarfism (Achondroplasia) D = allele for dwarfism d = allele for normal height Possible Genotypes: DD Dd dd Phenotypes: Dwarf stature Dwarf stature Normal height Solving Genetic Problems Faye has albinism (a recessive autosomal trait) while Fred has normal pigmentation. However, Fred's father had albinism. What is the probability that Faye and Fred will have a child with albinism? What is the probability that they will have a child that is a carrier of the albino allele? Start with determining the genotypes of the parents, then do the punnett square. Genetic Pedigree A pedigree is a family tree showing the genetic connections amongst the family members regarding a single gene. Pedigrees are especially useful in following recessive genetic disorders as there may be several family members that do not express the disorder but are carriers (heterozygotes). Pedigree for a Dominant Autosomal Trait AA Phenotype affected Aa aa Unaffected Phenotype Affected individuals have at least one affected parent The phenotype generally appears every generation Two unaffected parents (aa x aa) will only have unaffected offspring. Pedigree for a Recessive Autosomal Trait AA Aa Unaffected Phenotype aa Affected Phenotype Unaffected parents can have affected offspring Aa x Aa can give rise to: aa Practice Cystic Fibrosis (autosomal recessive disorder) Pedigree What are the possible genotypes of the third generation individuals of this family?  Levels of Dominance ot all genes are expressed as simply as the ones we have seen so far. Degrees of dominance can vary: Complete Incomplete Codominance Dominance Dominance Dominant allele Neither allele is expressed when completely genotype dominant. Both alleles homozygous Heterozygotes are expressed dominant or have a phenotype in the heterozygous. intermediate phenotype. Like Mendel’s between the 2  Complete Dominance The presence of the dominant allele eliminates the expression of the recessive allele. e.g. Recall the gene determining eye colour. The allele brown eyes is dominant over the allele for blue eyes. Therefore, heterozygous individuals will have brown eyes. Genotype: EE Ee ee Phenotype:  Incomplete Dominance Heterozygous individuals have a phenotype intermediate between the 2 homozygous phenotypes. Hair texture Allele S: Straight hair Allele C: Curly Hair Genotype: SC gives wavy hair Codominance Neither allele is dominant over the other. Both alleles are expressed phenotypically. Example: A,B, O Blood types  Controlled by a single autosomal gene which determines the type of identification markers added to the surface of red blood cells.  The gene actually has 3 possible alleles IA, IB, and i.  IA and IB are codominant. Meaning an individual with both alleles (heterozygotes) will have both types of identification markers on their red blood cells.  Multiple Alleles Most genes have more than 2 possible alleles. e.g. The ABO blood group gene has 3 possible alleles: IA IB i Note: Although there are 3 possible alleles for this gene, an individual will only have 2 of them. IA allele adds the A identification marker to the surface of red blood cells IB allele adds the B identification marker. i allele doesn’t add an identification marker. Alleles IA and IB are codominant Allele i is recessive to both IA and IB Practice: A man who has blood type A (heterozygous) is having a child with a woman who has blood type AB. What is the probability of this couple having a child with blood type B?  Sex Chromosomes Humans have 2 sex chromosomes Females have 2 X chromosomes. Males have one X and one Y. ex Chromosomes contain genes that:. Determine the biological sex of the individual. Control development of secondary sex characteristics. BUT also carry genes for other characteristics Sex Determination The sex chromosome in the sperm cell will determine the biological sex of the child. Mother’s gametes Father’s gametes X-linked Characters in Humans Characters determined by genes found only on the X chromosome. For genes that are present on the X chromosome: Dominance of alleles can only apply to females since they have 2 copies of the X chromosome and therefore 2 alleles for those genes. Males only have one X chromosome, therefore the allele they have will always be expressed. Because of this, males will express recessive deleterious X-linked traits more often than females. Daughters receive an X chromosome from each parent while sons only receive an X chromosome from their mothers. X-Linked Human Disorders Mutated allele version of a gene on the X Chromosome causes the disorder. Examples: Hemophilia (blood clotting disorder) Duchenne muscular dystrophy Colour blindness Androgen insensitivity syndrome Colour Blindness Hemophilia Lack protein clotting factor required to Duchenne Muscular Dystrophy (DMD) Affects 1 in every 3500 males born worldwide Caused by a faulty recessive allele for a gene on the X chromosome that is responsible for making a key muscle protein called dystrophin. Causes progressive weakening of muscles and loss of coordination. Life expectancy: early 20s Are you colour blind? What are the numbers in the images to the right. Colour Blindness Females XX Possible Genotypes Possible Caused by an Phenotypes allele on the X chromosome that XC X C normal vision is faulty leading XC X c normal vision but to reduced carrier for the numbers of cone trait cells in the eyes Xc X c colourXY blind Males and a weaker ability to discern Possible Genotypes Possible Phenotypes differences in XC Y normal vision colour. Xc Y colour blind Practice Question Fred is colour-blind (an X-linked recessive trait) while Anne has normal colour vision. However, Anne's sister is colour-blind. If they are expecting a son what is the probability that he will be colour blind? What is the probability that they will have a child that is a carrier for colour-blindness? Androgen Insensitivity Syndrome Caused by a recessive allele for the AR gene on the X- chromosome which causes the individual to not produce androgen receptors at all or have fewer than normal. Normal Testosterone signaling pathway Recall that testosterone is a hydrophobic (lipid soluble) hormone. Target cells produce the Androgen receptor (AR) that it needs to bind to in the cytoplasm to influence cell activity. Androgen Insensitivity Syndrome (AIS) Individuals are genetic males with one X and one Y chromosome, however, their cells cannot at all respond to testosterone (complete AIS) or have a reduced ability to respond to it (Partial AIS). Complete AIS Female external genitals so considered female at birth, however genetically male. Absence of a uterus and presence of testes that remain in the abdominopelvic cavity. No menstruation occurs and they are sterile Partial AIS Individua may have genitalia that look typical for females, genitalia that have both male and female characteristics, or genitalia that look typical for males. Nature and Nurture The environment in which we live and grow can affect our phenotypic expression of our genes. E.g. Inheritance of height in humans:  Gene sets limit (ex not 10 feet tall), but environment shapes person within his/her limits  Modified by variety of environmental conditions: diet, general health, stress, sleep, etc. Identical Twin Studies Identical twins have exactly the same genome but over time have different experiences and lifestyles. Ultimately these environmental differences can alter the expression of their genes. How does stress or other environmental factors affect the expression of our genes? Epigenetics Environmental changes can cause genes to be expressed differently from before. Molecules can be attached to our chromosomes that switch genes on or off changing our traits. Mental Health and Genetics Many psychological disorders seem to have a genetic basis. Family members may have a predisposition for developing the disorder. However, this doesn’t guarantee that they will, environmental factors such as stress, strongly influence the likelihood of them developing the disorder. A study published in the Lancet in 2013 genetically screened 33,000 patients with mental illness and 28,000 people who had no major psychiatric diagnosis. Analysis revealed a selection of genes that were significantly different (allele variation) for those with psychiatric diagnoses such as schizophrenia, bipolar and autism spectrum disorder. One of the functions of these genes is making membrane transport proteins that regulate flow of calcium ions into neurons. If flow of Ca2+ into neurons is abnormal, what aspect of neuron physiology would be affected? Diathesis-Stress Model Diathesis is another word for predisposition. The diathesis-stress model shows how genetic traits (diatheses) interact with environmental influences (stressors) to produce mental disorders such as depression, anxiety, or schizophrenia. Someone that has no predisposition for mental illness may withstand a significant amount of stress and never develop a mental illness. Whereas someone that has mental illness in their family and has inherited the genetic variations will be more prone to developing mental illness