More Variations to Mendel’s Laws

Mitochondrial Genes

Mitochondrial Inheritance Pattern Mitochondrial genes are passed from mothers to offfspring. Mitochondrial genes are passed from mothers to offfspring. Only females pass on the genes Only females pass on the genes

The 37 Mitochondrial Genes 24 encode proteins important for protein synthesis 24 encode proteins important for protein synthesis –Mutations can have devastating effects 13 encode proteins needed for energy production 13 encode proteins needed for energy production –Mutations often affect skeletal muscle and cause fatigue

Heteroplasmy A mutation can occur in one mitochondrial DNA ring and not another. A mutation can occur in one mitochondrial DNA ring and not another. When the mitochondria divide, different batches of daughter mitochondria are produced (some with the mutation, some without) When the mitochondria divide, different batches of daughter mitochondria are produced (some with the mutation, some without) It is therefore possible to have mutant mitochondrial DNA in some tissues but not others It is therefore possible to have mutant mitochondrial DNA in some tissues but not others Causes variation is expressivity of a mitochondrial disease depending on which tissues/organs have cells with mutated mitochondrial DNA Causes variation is expressivity of a mitochondrial disease depending on which tissues/organs have cells with mutated mitochondrial DNA

Linkage Two genes on the same chromosome may stick together Two genes on the same chromosome may stick together Example: Dihybrid cross of pea plants with purple flowers (Pp) and long pollen grains (Ll) Example: Dihybrid cross of pea plants with purple flowers (Pp) and long pollen grains (Ll)

Parents P Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross p Ll P L p l Figure 5.10

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents P Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross F1F1 p Ll P L p l

Parents P Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross F1F1 p Ll P L p l Female gametes PLPlpLpl Male gametes PL Pl pL pl Female gametes PLpl Male gametes PL pl

Parents P Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross F1F1 p Ll P L p l Female gametes PLPlpLpl PPLLPPLlPpLLPpLl PPLlPPllPpLlPpll PpLLPpLlppLLppLl PpLlPpllppLlppll Male gametes PL Pl pL pl Female gametes PLpl PPLLPpLl ppll Male gametes PL pl

Parents P Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross F1F1 Phenotypic ratio 3: Self-cross p Ll P L p l Female gametes PLPlpLpl PPLLPPLlPpLLPpLl PPLlPPllPpLlPpll PpLLPpLlppLLppLl PpLlPpllppLlppll Male gametes PL Pl pL pl Female gametes PLpl PPLLPpLl ppll Male gametes PL pl Phenotypic ratio 9:3

Parents P Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross F1F1 Phenotypic ratio 3: Self-cross p Ll P L p l Female gametes PLPlpLpl PPLLPPLlPpLLPpLl PPLlPPllPpLlPpll PpLLPpLlppLLppLl PpLlPpllppLlppll Male gametes PL Pl pL pl Female gametes PLpl PPLLPpLl ppll Male gametes PL pl Phenotypic ratio 9:3:3

Parents P Genotype PpLl Genes not linked Genotype PpLl Genes linked Self-cross F1F1 Phenotypic ratio 3:1 Self-cross p Ll P L p l Female gametes PLPlpLpl PPLLPPLlPpLLPpLl PPLlPPllPpLlPpll PpLLPpLlppLLppLl PpLlPpllppLl ppll Male gametes PL Pl pL pl Female gametes PLpl PPLLPpLl ppll Male gametes PL pl Phenotypic ratio 9:3:3:1

Crossing Over May Disrupt Linkage

Linkage Maps The frequency of recombination between two genes is proportional to the distance between the genes The frequency of recombination between two genes is proportional to the distance between the genes –i.e. The farther apart 2 genes are, the more likely their linkage will be disrupted during crossing over –Therefore, % recombination tells us the relative location of the genes

Linkage Maps

Sex Chromosomes

Autosome =a chromosome that does NOT contain a gene that determines sex Autosome =a chromosome that does NOT contain a gene that determines sex –i.e. any chromosome that is not a sex chromosome Humans have 22 autosome pairs and one pair of sex chromosomes

The Sex Chromosomes Heterogametic -Males have an X and a Y chromosome (XY) Homogametic -Females have 2 X chromosomes (XX) In other species sex can be determined in different ways – –For example, in birds and snakes males are homogametic ZZ females are heterogametic ZW

Sex Determination

The Y Chromosome Has 231 protein-encoding genes Has 231 protein-encoding genes –The X chromosome has >1500 genes Contains amplicons Contains amplicons –Palindrome-ridden regions –GACATACAG

The SRY Gene Sex-determining region of Y (SRY) Sex-determining region of Y (SRY) Encodes a transcription factor Encodes a transcription factor –A type of protein that controls the expression of other genes Leads to: Leads to: –Development of Wolffian ducts –Break down of Müllerian ducts –Secretion of testosterone

In early embryo (week 6)

X-linked and Y-linked Traits Genes carried on the sex chromosomes Genes carried on the sex chromosomes X-linked traits X-linked traits –In females, an X-linked trait is passed on just like an autosomal trait because there are a pair of X chromosomes 2 copies required for expression of a recessive trait 2 copies required for expression of a recessive trait Females get one X from mom and one X from dad Females get one X from mom and one X from dad –In males, only one copy of a recessive allele are needed Males are “hemizygous” because the genes on the X chromosome have no match on the Y chromosome Males are “hemizygous” because the genes on the X chromosome have no match on the Y chromosome Males get their X from mom Males get their X from mom Y-linked traits Y-linked traits –Very rare –Transmitted from father to son

X-linked Traits Possible genotypes X + Y  Hemizygous wild type male X m Y  Hemizygous mutant male X + X +  Homozyogus wild female X + X m  Heterozygous female carrier X m X m  Homozygous mutant female

X-linked Recessive Traits Always expressed in hemizygous males Female homozygotes show the trait but female heterozygotes do not Affected males: Inherited from affected or heterozygous mother Affected females: affected fathers and affected or heterozygous mothers