Sex influenced traits• The gene is NOT on a

sex chromosome, but SEX affects the phenotype

• Ex-baldness-dominant in males, recessive in women– If ‘B’ represents bald

and ‘b’ is hairy then Men must be bb to keep

hairWomen can be Bb or bb

to keep hair

• Caused by a gene that is located on a SEX chromosome (X or Y)

• Most SEX-linked Traits are found on the X chromosome

X-linked diseases

• Most are recessive• Examples-hemophilia,

red-green colorblindness• Males are more likely to

have these because they cannot be carriers

• Why?• Males are XY-if their ‘X’

has a bad gene, there is nothing on the ‘Y’ to dominate over it

How A Sex-linked trait is passedEx. Red-Green Colorblindness (x-recessive)

• N

n

• XNXN=Normal • XNXn=CARRIER, but

IS NOT colorblind• XNY=Normal• XnY=HAS red-green

colorblindess

Passing colorblindness

• Carrier mom XNXn

• Normal Dad XNY

XN Xn

XN XNXN XNXn

Y XNY XnY

X-linked genes are on the X chromosome

Autosomal genes-NOT on the X or Y chromosomes

Other X-linked traits

• Orange and Black Alleles are on the X chromosome in cats

• White is on another chromosome

• In every cell, only 1 X chromosome is active (other X is Barr Body)

• Result-some cells make black fur, some orange

Multifactorial and polygenic traits

• Polygenic-more than one pair of alleles determines phenotype- -eye color

• Multifactorial-genes AND environment determine phenotype –weight and height

Epistasis

• Multiple alleles (more than 2 involved)

• One allele causes the other to act differently)

• Example-E is brown unless B is present