Extensions to Mendel’s Laws Mendelian traits tend to be the exception Allele- alternate form of same gene Loci- location of a particular gene Chromosome- carries genes Chapter 5 (pp 95-113) I. Mendelian Ratios Wrong? a. Mendel’s Ratios i. Monohybrid - 3:1 ii. Dihybrid – 9:3:3:1 b. Lethal allele combinations (in humans many lead to “miscarriages”) i. any genotype that causes death ii. in a population sense it is more specifically any genotype that prevents passage of genes to the next generation EX. Mexican Hairless Dogs- Hairless is a dominant trait, Homozygous dominant is lethal c. Multiple alleles- (several hundred in humans) i. Humans have 2 alleles for all autosomal traits however the gene can exist in more than 2 forms ii. Leads to variation in phenotypes EX. ABO blood group d. Incomplete Dominancei. the heterozygous phenotype is intermediate to the both homozygotes EX: SnapdragonsHair CurlinessRR – Red CC- Curly rr – White cc – Strait Rr - Pink Cc - Wavy e. Codominanti. both alleles are expressed in the heterozygote EX. Coat Color in Horses Red and White both homozygous Roan (both red and white hairs) is heterozygous ii. ABO blood group 1. 4 blood types- A, AB, B, O 2. 3 alleles- IA, IB, i (IO) (I = isoagglutingen = antigen) Genotype IA i IA IA IA IB IB i IB IB ii Antigen Present A A A and B B B none Phenotype A A AB B B O 3. A and B are dominant to O, but Codominant to each other 4. Important for compatible blood transfusion f. Epistasisi. One gene effects the expression of another gene EX #1 – Gene for albinism would mask the gene for hair color Many different types of Epistasis that lead to some variation of the Mendel’s 9:3:3:1 ratio ii. Duplicate Recessive Epistasis1. must have the presence of 2 genes to express another 2. EX: must have B and C to express E or e 3. ratio is 9:7 iii. Dominant Epistasis4. presence of one gene masks the expression of the dominant and recessive forms of another 5. EX: B masks the expression of E and e (an entire trait) 6. ratio is 12:3:1 iv. Recessive Epistasis7. presence of one gene masks the expression of only the dominant form of another 8. EX: B masks the expression of E only (only dominant form of trait) 9. ratio is 9:4:3 g. Penetrance and Expressivity – explain the degree of gene expression i. completely penetrant - everyone who inherits the combination of alleles has some symptoms ii. incompletely penetrant - some individuals do not express the phenotype or have no symptoms 1. Use percentages to describe Penetrance numerically EX. Polydactyly- (extra fingers or toes) iii. variably expressive – intensity varies in different people 1. some people may have an extra digit on every extremity or some may just have a partial digit on one extremity h. Pleiotropy - one gene (protein) controls several functions or has more than one effect i. Phenocopy – an environmentally caused trait that appears to be inherited EX. an injury to the pancreas can mimic diabetes j. Genetic Heterogeneity – different genes can produce the same phenotype EX. 132 forms of deafness II. Mitochondrial Genes a. Mitochondrial genes only come from mother i. mothers usually pass all disorders to all offspring b. Only DNA fingerprinting that can be done with hard remains (bone / teeth) Trade Center 9/1/01! III. Linkage – transmission of genes on the same chromosome a. do not follow Mendelian ratios because genes do not independently assort b. Recombinant – a series of alleles on a chromosome that differs from the series of either parent i. a result of crossing over Sex Determination and Linkage (Chp. 6) I. Sexual Development a. Y chromosome has SRY – sex-determining region of the Y (1990) b. absence of SRY leads to female development i. Some males that are XX 1. one X has the SRY ii. Some females that are XY 1. Y chromosome lacks the SRY c. Sex Chromosomes not always X and Y i. Heterogametic sex- two different sex chromosomes (XY) ii. Homogametic sex – two of the same sex chromosomes (XX) iii. Birds1. males- ZZ 2. females - ZW d. Y chromosome (Fig. 6.3) i. is fairly simple for several reasons ii. very few genes – 85 genes iii. No homolog to crossover with iv. Y has 3 functional groups 1. pseudoautosomal regions (PAR1 and PAR2)- regions at the tips of the chromosome a. may cross over with regions of the X chromosome b. protein functions found in both sexes i. Bone growth, hormones… 2. X-Y Homologs- genes found on the Y that are very similar to the X but not identical as they are in the PAR 3. Genes that are unique a. SRY b. Sperm development e. X chromosome is much larger than the Y chromosome i. X – more than 1,000 genes II. Sex Linked Traits a. Y-Linked – on the Y chromosome i. Rare ii. Only one clearly defined is infertility (can’t be passed on) b. X-Linked – on the X chromosome i. in females passed on just like autosomal traits ii. In males it’s different because only one X exists iii. Males are Hemizygous- only one set of X-linked genes 1. X always comes from mom c. X-Linked Recessive i. always expressed in the male ii. Expressed in a female homozygote but not the heterozygote iii. Passed from heterozygote or homozygote mother to affected son iv. affected female has an affected father and a affected mother or a heterozygote mother v. EX: ichthyosis, color blindness, hemophilia d. X-Linked Dominant i. expressed in female in one copy ii. expressed more severely in male iii. high rate of miscarriage due to early lethality in males iv. EX: hypertrichosis (extra hair follicles) III. IV. X- Inactivation- Fig. 6.13 a. most of the genes on one X chromosome in each cell are inactivated b. which X (from mother or father) is inactivated is random c. females express traits from mother in some cells and traits from father in other cells d. Barr Bodyi. the inactivated X chromosome ii. only present in females e. Manifesting Heterozygotei. A carrier of an X-linked trait who expresses the phenotype ii. due to X-inactivation Gender Effects on Phenotype a. Sex-limited Traitsi. a structure or function of the body that is present in only males or only females ii. ex: horn development, milk yield, beard growth… iii. genes are transmitted by parents but hormones are not present to express trait b. Sex-influenced Traitsi. an allele is dominant in one sex and recessive in another 1. caused by hormonal differences 2. ex: male pattern baldness (Bb male = bald, bb female = bald) c. Genomic Imprinting- (parent of origin) – Fig. 6.15 i. difference in gene expression of a gene or chromosomal region depending upon whether it is inherited from the father or the mother and depending on male or female ii. women can have sons and men can have daughters without passing on their sex-specific parental imprints