24 and 26 Jan, 2005 Chapter 5 The Inheritance of Single- Gene Differences Alleles at single locus

Overview In matings, precise phenotypic ratios are produced in descendants as a result of chromosome segregation. In heterozygotes, alleles segregate equally into meiotic products. Progeny ratios can be predicted from known genotypes of parents. Parental genotypes can be inferred from phenotypes of progeny. In many organisms, sex chromosomes determine sex. X-linked genes can show different phenotypic ratios in male and female progeny. In humans, single-gene traits can be studied in pedigrees. Organelle genes are inherited maternally.

Meiotic chromosome segregation In meiosis, each of the four haploid products receives one of each kind of chromosome –A/A homozygotes  all get A chromosomes –A/a heterozygotes  half get A chromosomes half get a chromosomes As a consequence of chromosome segregation, alleles of heterozygotes segregate equally

Equal segregation First observed by Mendel in crosses with peas Readily observed using some fungi and protists in which all four haploid products of meiocyte (tetrad) can be analyzed Aa A aA a

Tetrads and Octads

Diploid crosses (1) Three possible diploid genotypes A/A a/aA/a Six possible diploid crosses CrossGenotypic ratioPhenotypic ratio A/A  A/A A/Aall A a/a  a/a a/aall a A/A  a/a A/aall A A/a  A/A 1A/A:1A/aall A A/a  a/a 1A/a:1a/a1A:1a A/a  A/a 1A/A:2A/a:1a/a3A:1a

Diploid crosses (2) Crosses between individuals heterozygous for the same single gene are also called monohybrid crosses A heterozygote for unexpressed recessive allele is sometimes called a carrier, particularly in humans A cross between an unknown genotype (e.g., A/–) and the homozygous recessive genotype (a/a) is called a testcross

Mendel

Testcross Distinguishes between A/A and A/a genotypes mated to a/a based on phenotypes of offspring if all progeny are dominant phenotype, then unknown is A/A if at least one offspring is recessive phenotype, then unknown is A/a If:genotypic outcomephenotypic outcome A/A  a/a A/aall A A/a  a/a 1A/a:1a/a1A:1a

Autosomes and sex chromosomes Sex chromosomes determine sex in most animals and some plants –usually only one pair –one sex has two alike (e.g., XX) –one sex has two different types (e.g., XY) Remaining chromosomes are called autosomes X chromosomes and autosomes contain numerous genes Y chromosomes typically have few genes

Sex chromosome inheritance in humans 46A XX is female, homogametic (only X gametes) 46A XY is male, heterogametic (X and Y gametes) Segregate equally into gametes at meiosis Sperm Eggs Gametes50% X50% Y 50% XXXXY 50% XXXXY

Sex-linked cross

Pedigrees (1) Analysis of inheritance in families Typically small number of offspring Mendelian ratios rarely observed Allow inferences concerning genotypes and predictions concerning phenotypes of offspring (genetic counseling) unaffected male unaffected female affected male affected female

Pedigrees (2) Two children, one of each sex, show the trait Conclusions: –must be autosomal recessive trait –parents must be heterozygous –2/3 chance that each unafflicted child is heterozygous

Categories of inheritance Autosomal recessive –e.g., PKU, Tay-Sachs, albinism Autosomal dominant –e.g., Huntington disease X-linked recessive –e.g., color-blindness, hemophilia X-linked dominant –e.g., hypophosphatemia Y-linked Organelle

Assignment: All Chapter 5 problems, especially 3, 4, 7-12, 15, 17, Continue with bioinformatics tutorial.