Complex Traits of Heredity

Chpt. 12

Recall Simple Types of Heredity

Recessive Recall, must have both recessive alleles (aa) to have a recessive trait

Autosomal (Complete) Dominance Recall, only 1 allele needed to have a dominant trait (AA or Aa)

Complex Patterns of Inheritance

Complex Traits

Simple Mendelian genetics does not explain most traits

For example: There are at least 8 different genes associated with eye color so it isn’t so cut and dry like Mendel’s pea plants!!

CodominanceBoth Alleles Expressed

Together

Below; Both Pigmented (Red) and Non-Pigmented (White)

Both the pink and the white alleles are expressed in the heterozygote**RR=Red **W W= White **RW (Heterozygous) ~~~displays BOTH Red AND white alleles!!

Codominance in Animals ** Example: When a red horse is crossed with a white horse, a roan horse is produced. The phenotypes are:

Red (RR) White (WW)

Occurs in cows too!!!

Roan (RW)

Heterozygous Red and White (Hybrid)

Codominant Punnett Square

Roan Cow (RW)

Roan Bull (RW)

R

W

R W

RR R W

R W WW

1Red:2Roan:1 White25% Red25% white50% Roan (red AND White)

Who’s the Father of the Calves….Codominance Mystery!!!

Incomplete Dominance – two phenotypes create a heterozygote that is intermediate between each of them; A blending of the

two phenotypes to create a third phenotype

RRR’R’

Heterozygous/Hybrid

Intermediate

R’R

R’R

WHITE REDPINK

USE PRIMES ABOVE THE ALLELE (LETTER) FOR LACK OF PIGMENTATION…

Incomplete Dominance

Straight HairH’H’

Wavy Hair Intermediate

PhenotypeH’H

(Heterozygous)

Curly HairHH

Incomplete Dominance – Hair

H

H’

H’

H H’HHH

H’H’H’H

A couple, both with wavy hair, want to have a baby. What are the genotypic and

phenotypic ratios of their possible offspring??

Genotypic Ratio:1HH:2H’H:1H’H’

Phenotypic Ratio:1Curly:2Wavy:1Straight

X-Linked (sex linked) Traits/Disorders/Conditions

X-linked TraitsIf a Recessive Trait: XNXN = Normal female XNXn = Normal CarrierXnXn Affected femaleXNY= Normal MaleXnY=Affected Male

Males can NEVER be carriers!!!!

Colorblindness Is Recessive and On the X Chromosome

Hemophilia: sex-linked recessive trait. If affected, lack a blood clotting factor.

May die from a cut if left untreated

Treatment entails giving the person the clotting factor by injection

X-linked – REMEMBER NO SUPERSCIPTS ON Y CHROMOSOME!!!

If a man is colorblind and has a child with a heterozygous female carrier………What is Dad’s genotype? Mom’s?

Xn Y

XN Xn

Xn

XN XN Y

Xn Xn Xn

Y

1. Probability of a Colorblind Daughter??

1 of 2 Daughters or 50% of the girls

2. Colorblind Sons??

Xn

YXN

Xn

1 of 2 Sons or 50% of the boys

3. Out of ALL of the children, how many are carriers??

ONLY 1 of the four Children… Why?

Males only have 1 X!! Can’t Carry on the Y….this is X-Linked!!!

Polygenic Traits – 2 or more genes affect one phenotype

Examples: hair color, eye color, height

**Polygenic traits follow a normal distribution, bell curve based on an average of a population

Notice the Difference in Brightness!! Light Colors to Dark Colors!!

A simplified model for polygenic inheritance of skin color

** Polygenic traits in a population typically follows a bell curve; lightest skin color to the left of the curve and the darkest to the right……Notice the majority of the population is a medium tone

Multiple Alleles – 3 or more forms of a genes (alleles)

involved in ONE trait

In the hypothetical family above, there are 6 alleles for one trait

Blood Types are controlled by three alleles….…A,B and O

Blood Types – Multiple Alleles

Phenotype Genotype

Type AB IAIB , or AB Codominant!!

Type O ii or OO Recessive

Type A IAIA or AA (homozygous A)or

IAi or AO (heterozygous A)

Type B IBIB or BB (homozygous B)or

IBi, BO (heterozygous B)

Example of a punnett square of a Heterozygous Type A father and Type O mother.

Practice Punnett Squares – Cross the Following:

A homozygous Type A female with a Type O male

A Type AB male with an O female

A heterozygous Type B male with a heterozygous A female

A couple that are both type AB

Environmental Influences and

Genetics

External Factors and Internal

Factors• Temperature, nutrition, light, chemicals, hormones, age, can influence gene expression.

External Factors• In arctic foxes

temperature has an effect on the genetic expression of coat color seasonally.

External Factors• Leaves can have different sizes, thicknesses,

and shapes depending on the amount of light they receive.

Internal Factors• Males and females differ in hormones and

structural differences• can cause a single genotype to

express more than one phenotype (antlers in males)

Internal Factors• An organism’s age can also affect gene

function.• Random mutations can accumulate• Organ function diminishes