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More Variations to Mendel’s Laws Mitochondrial Genes Mitochondrial Inheritance Pattern • Mitochondrial genes are passed from mothers to • offfspring. Only females pass on the genes The 37 Mitochondrial Genes • 24 encode proteins important for protein synthesis – Mutations can have devastating effects • 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. 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 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 • Example: Dihybrid cross of pea plants with purple flowers (Pp) and long pollen grains (Ll) Parents P p L l Genotype PpLl Genes not linked Self-cross Figure 5.10 P L p l Genotype PpLl Genes linked Self-cross Parents P p L l Genotype PpLl Genes not linked Self-cross P L p l Genotype PpLl Genes linked Self-cross F1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents P p L P L l Genotype PpLl Genes not linked Self-cross F1 Male gametes Genotype PpLl Genes linked Self-cross Female gametes PL pl Female gametes PL Pl pL pl PL Pl pL pl p l Male gametes PL pl Parents P p L l Genotype PpLl Genes not linked Self-cross F1 Female gametes PL Pl pL pl PL Pl Male gametes pL pl P L PPLL PPLl PpLL PpLl PPLl PPll PpLl Ppll PpLL PpLl ppLL ppLl PpLl Ppll ppLl ppll p l Genotype PpLl Genes linked Self-cross Female gametes PL pl Male PL PPLL PpLl gametes pl PpLl ppll Parents P p L l Genotype PpLl Genes not linked Self-cross F1 Female gametes PL Pl pL pl PL Pl PPLL PPLl PpLL PpLl PPLl PPll PpLl Ppll Male gametes PpLL PpLl ppLL ppLl pL pl PpLl Ppll ppLl ppll Phenotypic ratio 9:3 P L p l Genotype PpLl Genes linked Self-cross Female gametes PL pl Male PL PPLL PpLl gametes pl PpLl ppll Phenotypic ratio 3: Parents P p L l Genotype PpLl Genes not linked Self-cross F1 Female gametes PL Pl pL pl PL Pl PPLL PPLl PpLL PpLl PPLl PPll PpLl Ppll Male gametes PpLL PpLl ppLL ppLl pL pl PpLl Ppll ppLl ppll Phenotypic ratio 9:3:3 P L p l Genotype PpLl Genes linked Self-cross Female gametes PL pl Male PL PPLL PpLl gametes pl PpLl ppll Phenotypic ratio 3: Parents P p L l Genotype PpLl Genes not linked Self-cross F1 Female gametes PL Pl pL pl PL PPLL PPLl PpLL PpLl Pl PPLl PPll PpLl Ppll Male gametes pL PpLL PpLl ppLL ppLl pl PpLl Ppll ppLl ppll Phenotypic ratio 9:3:3:1 P L p l Genotype PpLl Genes linked Self-cross Female gametes PL pl Male PL PPLL PpLl gametes pl PpLl ppll Phenotypic ratio 3:1 Crossing Over May Disrupt Linkage Linkage Maps • 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 Sex Chromosomes • 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 – The X chromosome has >1500 genes • Contains amplicons – Palindrome-ridden regions – GACATACAG The SRY Gene • Sex-determining region of Y (SRY) • Encodes a transcription factor – A type of protein that controls the expression of other genes • 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 • 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 • 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 get their X from mom • Y-linked traits – Very rare – Transmitted from father to son X-linked Traits Possible genotypes X+Y Hemizygous wild type male XmY Hemizygous mutant male X+X+ Homozyogus wild female X+Xm Heterozygous female carrier XmXm 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