Genetics:

1
Genetics

• Beyond Mendel

2

• Incomplete Dominance

• Neither allele is dominant, blending of both
  alleles to create a different phenotype.
• Example If snapdragons have a red flower
  allele and a white allele, flowers are pink.

3
Representing Incomplete Dominance

• Since neither allele is dominant, alleles with
  incomplete dominance are represented as a
  superscript above a letter representing the gene.
• Example Red color CR
• White color - CW

4
Codominance

• Both alleles are expressed in a heterozygote.
• Example Roan cattle with mixed red and white
  hairs are heterozygotes for red and white hair
  color
• alleles.

5
Representing Codominance

• As with incomplete dominance, codominance is
  represented with a superscript.
• Example red CR, white CW
• The difference between incomplete dominance
  and codominance is subtle, and they can be
  treated the same.

6
Applying the Concepts

• Which produces the greatest number of roan (CRCW)
  offspring?
• Roan x Roan (CRCW x CRCW)
• OR
• Red x White (CR CR x CWCW)

CR CW CR CW
CRCR
CRCW
CRCW
CWCW
CR CR CW CW
CRCw
CRCW
CRCW
CRCW
ALL ROAN
7

• For incomplete dominance, the phenotype ratio is
  the same as the genotype ratio.
• A heterozygote has a different phenotype than
  either homozygote in incomplete dominance, so
  there are three possible phenotypes.

8
Multiple Alleles

• For some traits, more than two possible alleles
  exist.
• Example 3 Blood Type Alleles
• A ( IA)
• B (IB)
• O (i)
• A and B are codominant
• O is recessive

9

• Blood type AB has both marker proteins (A and
  B) on red blood cell surface.

10
Blood Type (Phenotype) Possible Genotype(s)
A
B
AB
O
IAIA or IAi
IBIB or IBi
IAIB only
ii only
11
Paternity Tests

• A mother has blood type A, and her child has
  blood type O. Can a man with blood type B be the
  father?
• Yes, the mother can be IAi and the father can be
  IBi.
• A mother has blood type B and her child has
  blood type O. Can a man with blood type AB be
  the father?
• No, the child must have received an i allele
  from each parent.

12
Gender Determination

• In humans, the 23rd pair of chromosome (X/Y)
  are the sex chromosomes which determine the
  gender.
• Men are XY
• (X and Y are the only nonhomologous pair-
• which means that the pair does not contain the
  same type of gene.)
• Woman are XX
• The other 22 pairs of chromosomes are called
  autosomal chromosomes.

13
FEMALE
MALE
14
Autosomal Chromosomes
Sex Chromosomes
15
Sex Linked Traits

• Traits for which the gene is found on the X
  chromosome.
• Since men are XY, they have only one copy of
  sex-linked genes.
• Examples
• Red-Green Colorblindness
• Hemophilia

16

• Colorblindness Test

17
Sex-Linked Traits VideoClick once on image to
start video
https//www.youtube.com/watch?vH1HaR47Dqfw
18

• http//www.accessexcellence.org/HHQ/qow/qow09/qow0
  90810.php

19
Applying the Concepts

• Colorblindness is caused by a recessive allele
  on the X chromosome (Xb).
• If a colorblind woman(XbXb) marries a man with
  normal vision (XBY), what is the probability of a
  daughter being colorblind? A son being
  colorblind?

Xb
Xb
XBXb
XbXb x XBY
XBXb
XB
Daughter being CB O
XbY
XbY
Son being CB 100
Y
20
Wording of Probability in Sex-Linked Trait
Questions

• If the question asks What is the probability of
  the parents having a daughter that has normal
  color vision- it is not a given that the child
  is a daughter, so divide by all 4 possibilities
  2 out of 4 50
• If the question asks What is the probability of
  a daughter having normal color vision, it is a
  given that the child is a daughter. Only look at
  the 2 daughter possibilities 2 out of 2 100

21
Inheritance of Sex-Linked Traits

• A male can inherit colorblindness if his
  mother is a carrier of colorblindness.
• (Doesnt matter fathers color vision, because
  males inherits Y from dad).
• A female must inherit the colorblindness allele
  from both parents.
• Mother must be at least a carrier.
• Father must be colorblind.

22
Polygenic Traits

• Traits that are controlled by multiple genes.

Examples Human Height Skin Color Traits exhibit
a greater range of variation