GENETICS NonMendelian Genetics Linked Genes n Genes for

GENETICS Non-Mendelian Genetics

Linked Genes n Genes for 2 traits are located on the same chromosome and travel together into the same gamete P L p l

Crossing Over n New combinations of alleles in the offspring that occur when neighboring segments of tetrads break off where they meet and exchange genetic material

Sex-Linked Genes X-linked Genes: genes found only on the X chromosome n Y-linked Genes: genes found only on the Y chromosome n

X-Linked Genes n n Many hereditary diseases are X-linked, meaning the gene is found on the X chromosome Ex: red-green color blindness hemophilia x x x XX XX Y XY XY XX= normal female XX= carrier female XY= normal male XY= affected male

Test for Red-Green Color Blindness

Sex-Limited Traits n n Expressed only in the presence of sex hormones and are only observed in one sex or the other Controlled by genes not found on the sex chromosomes Although both males and females have these genes, they are only expressed by one sex since the appropriate sex hormone must be present EX: color in birds beard growth in human males

Sex-Influenced Traits Expressed in both sexes, but they are expressed differently n EX: Baldness n ¨ In the presence of the male sex hormone, the allele is dominant ¨ In the presence of the female sex hormone, the allele is recessive

Intermediate Inheritance The heterozygous offspring have traits that are not exactly like the trait of either purebred parent n There are 3 types of intermediate inheritance: n ¨ Incomplete Dominance ¨ Codominance ¨ Polygenic Inheritance

Incomplete Dominance n n In the heterozygous offspring, the dominant allele does not completely mask the effect of the recessive gene The offspring shows a phenotype that is in between the phenotype of the two homozygous parents

Example of Incomplete Dominance

Codominance n n Both alleles in the heterozygote express themselves fully Ex: Blood Types ¨ The IA & IB genes are both dominant, so if an individual has one of each of these genes, their blood type is AB ¨ The i. O gene is recessive n EX: Feather Color ¨ White (WW) and Black (BB) are both dominant, so a cross between a white chicken and a black rooster will produce a speckled offspring (BW)

Inheritance by Multiple Alleles n n n When there are more than two alleles in a population, these genes are said to have multiple alleles EX: blood type—there are 3 alleles in the population: IA, IB & i. O Possible combinations: Possible Genotypes IA IA IA i O IB IB IB i O Blood Types Type A Type B IA IB i. O Type AB Type O

Polygenic Inheritance n n Some traits are controlled by 2 or more genes EX: eye color

Pleiotrophy When a single gene affects more than one trait n EX: sickle cell anemia n ¨ Changes the cell shape ¨ Causes anemia ¨ Causes brain damage ¨ Causes spleen damage ¨ Causes heart damage

Body Temperature and Gene Expression n n In some organisms, body temperature can affect the expression of genes EX: Himalayan rabbit’s coat color ¨ The colder body parts (i. e. feet, ears, nose) are black ¨ The warmer body parts are white