Incomplete and Co Dominance

& Sex Linked Traits

Mr. Fernandes

Review of Mendel’s Principles

Genes are passed parents offspring;

get one allele from each parent

During Meiosis, the alleles for a gene

segregate from each other.

During Meiosis, genes independently

assort with each other.

Exceptions to Mendel’s principles

Sometimes, there is no dominant or

recessive gene, or the trait is controlled

by many alleles or genes.

Karyotype: a picture of chromosomes

Body Cells vs. Sex Cells

Autosomes: the first 22 homologous pairs of chromosomes.

Autosomes are the same for both males and females.

Sex cells: Determine sex of offspring

Is it Male or Female? Explain

Sex chromosomes: determines the sex of the individual

The sex chromosomes are the 23rd pair of chromosomes.

XX =female

DAD

Why?

All moms have the genotype XX. When egg cells are made, they will all carry a single X chromosome.

All dads have the genotype XY. When sperm cells are made, 50% will have an X chromosome and 50% will have a Y chromosome.

Therefore, males and females are born in roughly a 50:50 ratio.

Which parent determines the sex of an offspring?

SEX-LINKED TRAITS

Those traits that are controlled by genes on the X or Y chromosomes.

NOTE: The Y chromosome is much smaller than the X chromosome and only contains a few genes. Most sex-linked traits are on the X chromosome.

In humans, hemophilia is a sex-linked trait. Having

hemophilia is recessive (Xh) to being normal (XH). The

heterozygous female is called a carrier. Cross a carrier

female with a normal male.

__ XHXh __ X __ XHY ___ XH Xh

XH

Y

XH XH

XH Y Xh Y

XH Xh

male

Now You Try

Cross a carrier female with a male with hemophilia.

__ XHXh __ X __ XhY ___

In Incomplete Dominance, every genotype has its own

phenotype. (One allele not completely dominant over the

other.) Third phenotype that is a blending of the parental

traits. (2 alleles produce 3 phenotypes.)

Result: Heterozygous phenotype somewhere in

between homozygous phenotype.

Incomplete Dominance

1. Incomplete Dominance

Examples:

Trait: Flower Color

Expressions: Red x

White Pink

RR= Red; RW= pink; WW=

white

straight hair, wavy, curly

Incomplete Dominance

In codominance, neither allele are dominant;

both are expressed. A cross between

organisms with two different phenotypes

produces offspring with has both phenotypes

of the parental traits shown.

Codominance

2. Codominance Both alleles contribute to the phenotype.

Example: In come chickens

Black Chicken x White Speckled Chicken

YOU tell me which type of

dominance…

Codominance!

Type of

Dominance?

Incomplete

Dominance!

Type of Dominance?

Incomplete Dominance!