incomplete dominance codominance sex linked and polygenic inheritance
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Incomplete Dominance, Codominance, Sex-linked and Polygenic Inheritance. Mendel’s Dominance. Mendel’s rule of dominance was complete dominance Homozygous dominant organisms have the dominant phenotype Heterozygous organisms have the dominant phenotype

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mendel s dominance
Mendel’s Dominance
  • Mendel’s rule of dominance was complete dominance
    • Homozygous dominant organisms have the dominant phenotype
    • Heterozygous organisms have the dominant phenotype
    • Example: Both PP and Pp plants have the dominant PURPLE phenotype (P=purple and p=white flowers)
incomplete dominance
Incomplete Dominance

All alleles are dominant for the trait but different.

Offspring have an appearance somewhat in betweenthephenotypes of the two parents

“Mixed”

Blended

R

R

RR = pure red flower

WW = pure white

F1 generation

W

RW

RW

W

All RW = pink

(heterozygous pink)

RW

RW

incomplete dominance1
Incomplete Dominance

Incomplete dominance occurs when two or more dominant alleles are present.

Neither allele is completely dominant

over the other.

The offspring produced

are a blending of both

alleles.

P = RR (red) x WW (white)

F1’s = 100% RW (pink)

incomplete dominance2
Incomplete Dominance
  • F2 Generation
  • RW X RW
  • (pink) (pink)

RW

Offspring

25% - WW, white

50% - RW, pink

25% - RR, red

RW

WW

RW

RW

RR

codominance
Codominance

All alleles are dominant for the trait but

have a different phenotype.

BOTH allelesare expressedequallyinheterozygous individuals.

Neither allele is dominant over the other.

slide7

RR

Codominance example:

RR = pure red bull

WW = pure white cow

WW

RW

All offspring have the genotype = RW

Phenotype = All offspring are roan cows.

Roan = a mix of red & white hairs.

slide8

Codominance example #2

White hen Black rooster

WW

BB

W

W

X

B

BW

BW

B

BW

BW

BW

F1’s

Genotypes = 100% BW

Phenotypes = 100% checkered

Checkered chicken

slide9

Multiple Alleles

  • When there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population.
  • There may be multiple alleles within the population, but individuals have only two of those alleles.
  • Why?Because you have only two biological parents. 
  • Half of your genes (alleles) from Pa, & the other half from Ma, so you end up with two alleles for every trait in our phenotype.
slide10

BLOOD TYPES

  • Human blood type is an example of multiple alleles.
  • Blood exists as four possible phenotypes: A, B, AB, & O.
  • There are 3 alleles for the blood gene BUT remember,
  • you only have 2 of them 1 from Ma & 1 from Pa.
  • * The alleles are as follows:

Note:

*The allele for "O" (i) is recessive.

*The alleles for "A" & "B“ are dominant.

slide11

BLOOD TYPES cont’d

With three alleles we have a higher number of possible combinations in creating a genotype.

* There are 6 different genotypes & 4 different phenotypes for blood type.

  • "A" & "B" blood can be either homozygous (IAIA or
  • IBIB) or heterozygous (IAi or IBi), with one recessive
  • allele for "O“.
  • "O" blood is homozygous recessive (ii).
  • What\'s the deal with "AB" blood?
slide12

AB blood is codominant!

The "A" trait & the "B" trait appear together in the phenotype.

multiple allele codominance problem

IB

IB

IAIB

IAIB

IA

1/2 = IAIB

1/2 = IBi

i

IBi

IBi

Multiple Allele & Codominance Problem
  • Example:homozygous male Type B (IBIB)

x heterozygous female Type A (IAi)

another multiple allele codominance problem

IA

IB

IAi

IBi

i

50% = IAi, type A.

50% = IBi, type B.

i

IAi

IBi

Another Multiple Allele & Codominance Problem

Example: A woman with type O blood and a man with type AB blood are expecting a child.What are the child’s possible blood types?

multiple alleles codominance
Multiple Alleles & Codominance
  • Question:If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?

boy - type O (ii) X girl - type AB (IAIB)

Hint: You have to work this one backwards. Fill in what you know to find the parents.

multiple alleles codominance1

IA

i

IAIB

IB

i

ii

Multiple Alleles & Codominance
  • Answer:

girl

Parents must be:

genotypes = IAi and IBi

phenotypes = A and B

boy

slide17

Sex-Linked Traits

  • These are recessive traits located on the
  • sex chromosomes, #23.
  • Females genotype is XX.
  • Male genotype is XY.
  • Many of these traits are found on the X chromosome.
slide18

Sex-Linked Traits

  • Sex chromosomes are non-matching
  • chromosomes in males & females.
  • Whatever is on the X chromosome, the
  • male will get! There is nothing on the
  • Y chromosome to cover it up!
  • For females to get these recessive traits, the trait must be present on both X chromosomes.
  • When a female has the trait on one X chromosome, she is a carrier and can pass the trait on to her son.
sex linked traits

X

X

X

Y

Sex – Linked Traits
  • Example: Colorblindness
    • A mother carries the colorblindness gene on her X chromosome and has a 25% chance of passing on the disorder to her son.
    • X Y = colorblind male.
    • X X = carrier female.

c

XcX

X X

c

c

c

X Y

X Y

sex linked traits1
Sex-linked Traits

Example 2: Colorblindness

A colorblind father and a carrier mother are having child. Will their child be colorblind?

Probability:

25% colorblind female, XX

25% colorblind male, X Y

25% normal female, XX

25% normal male, XY

*Mom gives her bad X to her son.

*Dad gives his bad X to his daughter.

c

X

X

c

c

c

c

c

c

c

X

X X

X X

c

Y

X Y

X Y

p o l y g e n i c inheritance
Polygenic Inheritance
  • Inheritance pattern controlled by two or more genes
  • Genes can be on the same chromosome or on different chromosomes
  • Genotypes written as AA, Aa, or aa except you will have three or more genes each with a dominant and recessive trait
  • Skin color is influenced by the additive effects of melanin by three to six genes. Eye color is also a result of melanin added from multiple genes.
polygenic inheritance of skin color

Gametes

ABC

ABc

AbC

Abc

aBC

aBc

abC

abc

ABC

AABBCC

AABBCc

AABbCc

AaBBCC

ABc

AbC

Abc

AAbbcc

aBC

aaBBCc

aBc

abC

abc

aaBbcc

Polygenic Inheritance of Skin Color

The table shows typical ranges of skin color combinations. Other polygenic influenced conditions include Spina bifida and Cleft palate.

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