Genetics All somatic cells contain 23 pairs of chromosomes 22 pairs of autosomes 1 pair of sex chromosomes Genes contained in each pair of chromosomes

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1 Chapter 6 Genetics and Inheritance Lecture 1: Genetics and Patterns of Inheritance Asexual reproduction = daughter cells genetically identical to parent (clones) Sexual reproduction = offspring are genetic hybrids Tendency to inherit best traits of both parents Survival advantage against environmental change, competition, disease, etc. Siblings will often look similar, but not identical Each inherits 50% from each parent, but not the same 50% Crossing over Ultimate sources of variability Mutations Crossing over (recombination) Independent assortment Problem with inbreeding Limited number of genes Increased chances that deleterious mutations will show up Remember how mutations affect genes Protein product altered in 1 of 4 ways 1) No effect Silent mutation Mutations 2) Protein is altered, but it doesn t matter Neutral change HAT vs CAP 3) Protein loses some or all of its function Deleterious change - HAT vs CAT 4) Protein functions better Example: HIV resistance 1

2 Genetics All somatic cells contain 23 pairs of chromosomes 22 pairs of autosomes 1 pair of sex chromosomes Genes contained in each pair of chromosomes are identical Allele: Any form of a given gene in the population Humans are diploid Genetics For any given gene, we carry 2 alleles Homozygous: Both alleles are the same for a given gene Heterozygous: 2 different alleles for a given gene Heterozygosity 2 alleles for a given gene Each codes for a slightly different protein Which will be made? Both? Dominant One allele is usually chosen over the others Consistently chosen across the species Called the dominant allele Need only be present in one copy to be expressed Recessive Consistently ignored alleles are recessive Only expressed if present in 2 copies Can be passed on to offspring, even if not expressed Recessive does NOT mean rare, or even less common! Describes both alleles present for a given gene Capital letter = dominant Lower case letter = recessive Homozygous dominant = AA Heterozygous = Aa Genotype Homozygous recessive = aa 2

3 Phenotype Genotype is useful scientifically/medically, but what does the organism look like? Phenotype describes observable characteristics based on expression of the genotype Homozygous dominant = AA = brown eyes Heterozygous = Aa = brown eyes Homozygous recessive = aa = blue eyes Gregor Mendel Much of what we know about patterns of inheritance started with experiments done by this man Mendel observed 7 characteristics let s just look at seed color Examined patterns of inheritance of phenotype Experiment: cross plant with yellow seeds by plant with green seeds Result: all offspring had yellow seeds Y G Parent Y Y Y Y F1 Experiment: self-pollinated one of the new yellow-seeded plants Result: 25% of new plants had green seeds! Experiment: self-pollinated all of the F2 generation F2 Y F1 F2 Y Y Y Y 4:0 3:1 G G G G 0:4 3:1 F3 3

4 1. Factors for traits come in pairs only one will be passed from parent to offspring Carry 2 alleles for each gene Separated during meiosis Inherit one allele from each parent 2. If factors are identical, only that factor can be passed to offspring Homozygous 3. If factors are different, there is a 50/50 chance of each trait being passed on Heterozygous Another of 4. Some factors are inherited as a group, others are inherited randomly When genes are on the same chromosome, they are often inherited together Chromosomes are sorted randomly, so genes on different chromosomes are not inherited together (More on this later) Punnett Square Once diploidity was discovered, Mendel s observations were easily explained Punnett Square: a box diagram used to determine the probability of a given genotype Yellow seed color = dominant allele Green seed color = recessive allele Enough with the peas! Punnett Squares Let s look at a human disease : Tay-Sach s Disease (TSD) Autosomal recessive Affects the enzyme hexosaminidase A Lysosomal enzyme Point mutation in exon 11 that causes it to be cut out during splicing Fatty substance builds up in brain Mental, physical deterioration; death by age 4 4

5 Punnett Squares Scenario: 2 healthy individuals have a child with Tay-Sach s Autosomal recessive disease so child must be homozygous One allele inherited from each parent, yet each parent is healthy Both parents must be heterozygous We call these individuals carriers Have the disease gene, but do not have the disease Pedigrees A phenotypic family tree Used to determine genotype and track recessive alleles Females are circles Males are squares Darkened individuals have the condition being tracked Pedigrees Step 1: Assign a genotype to all affected individuals (darkened individuals) Step 2: Give all unaffected individuals one D Step 3: Give all offspring of affected individuals one d Step 4: Work backwards look at affected individuals; d must be present in both parents Step 5: Double check, but some will remain a mystery Some might be carriers, but no way to know from a pedigree 5