Blood Type and Sex Linked Inheritance

Multiple AllelesMultiple alleles for a particular gene means that

more than two alleles exist in the population.This does not mean that the gene in a particular

individual possesses more than two alleles. An individual can only have a maximum of

two of alleles, one maternal and one paternal, no matter how many alleles exist in the population.

Example: a population of dogs can have 5 alleles for coat color at one gene.

The Classic Example of Codominance and Multiple Alleles

in Humans is BLOOD TYPE

Gene that controls ABO blood type codes for an enzyme that makes an antigen on blood cells.Two alleles (IA and IB) are called “A” and “B” which are codominant. A and B can both be expressed on the same cell. Third allele (i) is called “O”) and it is recessive to both A and B

Possible Genotypes for Blood Types

If the Phenotype: A

Genotype(s): AA, Ao

If the Phenotype: B

Genotype(s): BB,Bo

If the Phenotype: AB

Genotype(s): AB

If the Phenotype: O

Genotype(s): oo

Possible combinations of alleles

A Real Problem:Mom is Type B, Dad is Type O … Baby is

Type AB. Is this possible?• Cross: ____ X ____Bo

MOMoo Remember,

when in doubt, heterozygous is more likely

B o

o

o

Bo

Bo

oo

oo SORRY! NOT

POSSIBLE

!

Check your understanding

Could an AB father have a type O child?Why?

Are the father’s alleles

codominant or recessive?

O is homozygous recessive. A or B would hide the o

allele.

Sex Chromosomes:the 23rd pair in humans

All others (1-22)

are called

autosomes

Boy Girl

The Sex Chromosomes

The “X” ChromosomeHas essential genesCan’t live without it!Females have a 23rd pair = XX (one usually deactivated)

The “Y” ChromosomeHas few genesMale 23rd pair = XY

What’s the probability of having a boy or girl?

XX

XY

XX

XY

X X

Y

X

50%

50%

Some X-Linked Traits:Hemophilia (missing clotting enzymes)

Bleeding disorders in which it takes a long time for the blood to clot.

ColorblindnessDefect in color vision

“Male-pattern” baldnessMost are recessive.

A man with normal vision marries a colorblind woman. What is the probability

that they will have a colorblind SON?

• Cross: ____ X ____XcXcMOM

XY Remember, Mom only shows if she is homozygous!

Xc Xc

X

Y

XcX

XcY

XcX

XcY

Focus only on SONS(not daughters)!

100%

(2/2) SONS

A hemophiliac man marries a healthy woman. What is the probability that they will have any

hemophiliac children?

All sons healthy

All daughters carriers

0%

ColorblindnessRed-green colorblindness-

Red and green appear to be the same. X-linked

Red-green affects:1 in 10 males in the US 1 in 100 females in the US

Remember: Males have just one X chromosome. All X-linked alleles are expressed in males.Other forms:

Total Colorblindness

Blue-Yellow colorblindness

Blue-yellow appears to be the same. It is autosomal Chromosom

es 1 to 22

What do you see?

No.Va.CRIME SCENE INVESTIGATON

http://www.ansci.cornell.edu/usdagen/mult_alleles.html

Medline Plus, 2014, US National Library of Medicine, National Institutes of Health http://www.nlm.nih.gov/medlineplus/ency/article/002051.htmVirtual Fruit Fly Lab, 2014, http://www.mhhe.com/biosci/genbio/virtual_labs/BL_15/BL_15.html