12.2 Notes

Special Inheritance

Autosomes--22 pairs of chromosomes in a human

Sex chromosomes--23rd (last) pair of chromosomes Ex. XX-female XY-male

The male always determines sex of offspring

22 Autosomes Sex chromosomes

Sex-linked traits--traits located on sex chromosomes Ex. In drosophila (fruit flies), XR = red eyes,

Xr = white eyes (located on X chromosome)

Cross white eye male (XrY) with red eye female (XRXR)

Xr Y

XR

XR

XRXr XRY

XRXr XRY

Phenotypic ratio--50% male 50% female 100% red eyes

White-eyed male (XrY)

Red-eyed female (XRXR)

Offspring: All red eyed

Cross red eye male (XRY) with red eye female (XRXr)

XR Y

XR

Xr

XRXR XRY

XRXr XrY

Phenotypic ratio--Female: 100% red-eye Male: 50% red-eye 50% white-eye

White-eyed male (XRY)

Red-eyed female (XRXr)

Simple Mendelian Inheritance--Inheritance controlled by either dominant or recessive alleles Not all traits are either dominant or

recessive, some are in between

Incomplete dominance—the phenotype of the heterozygous individual is an intermediate (blend) of the 2 homozygous individuals (neither allele is dominant)

RR= red flower WW= white flower RW(heterozygous) = pink flower

R R

W

W

RW RW

RW RW

Cross red with white--The offspring are all pink because that color is in between 2 homozygous phenotypes

Red (RR)

White (WW)

Offspring: All pink flowers

W

W

RW

RW

RW

RW

Co-dominance--The phenotype of both homozygous individuals to be present in the heterozygous individual

BB = black chicken WW= white chicken BW (heterozygous) = black and white chicken

B B

W

W

BW BW

BW BW

Cross black with white chicken--The offspring are all black and white because that is a combination of both homozygous phenotypes

Blood type An example of co-dominance in humans Example of multiple alleles, there are

more than 2 alleles for this trait It is determined by the presence or

absence of proteins (chains of amino acids) on the surface of blood cell

Mixing incompatible blood types can cause the cells to clump together, which can cause a person to die

Human Blood Types

Phenotype

Genotype Blood cell surface molecules

Type A IAIA or IAi A molecule

Type B IBIB or IBi B molecule

Type AB IAIB A and B molecules

Type O ii No molecules

Alleles IA and IB –are co-dominant to each other

Allele i –is recessive to both IA and IB

Type O blood—universal donor Has no proteins on the blood cells so any blood

type can receive it Type AB blood—universal acceptor

Has both A and B proteins on blood cells so this blood type can receive type A, B, AB, or O blood

Cross parent with A ( IAi) blood with a parent with B blood (IBi)

IA i

IB

i

IAIB IBi

IAi ii

Genotypic ratio

1 IA IB : 1 IA i : 1 IB i : 1 ii Phenotypic ratio (blood type)—1 type AB : 1 type A : 1 type B : 1 type O

Multiple alleles— traits controlled by more than 2 alleles Ex. Mouse hair color is controlled by

different alleles, black, brown, gray, albino, etc…

Polygenic inheritance--Trait that is controlled by 2 or more different genes

Ex. Blood pressure is controlled by genes for weight, cholesterol, kidney function, etc…