Genetics: Day 5

Pedigree Charts

• Pedigree charts are diagrams constructed to show biological relationships and in genetics are used to show how a trait passes from one generation to another

• Pedigree Key:

Pedigree Analysis

Determine the pattern of inheritance for the chart? Is it sex-linked dominant, sex-linked recessive, autosomal dominant, or autosomal recessive?

Let’s work this one out together on the board.

Dihybrid Crosses

• Used to determine potential genotypic and phenotypic ratios from parents using two characteristics

• Assuming the alleles are located on different chromosomes and not “linked” then we can assume the dihybrid cross will also follow Mendel’s Law of Independent Assortment

Example

Mrs. Parrott wants to predict whether her kids with Mr. Parrott will have straight hair, wavy hair or curly hair and dimples or no dimples. Below are their alleles.

Mrs.- wavy hair (Hh) and no dimples (dd)

Mr. - straight hair (hh) and dimples (DD)

hD hD hD hD

Hd HhDd HhDd HhDd HhDd

Hd HhDd HhDd HhDd HhDd

hd hhDd hhDd hhDd hhDd

hd hhDd hhDd hhDd hhDd

Genotypic ratio: 1:1 (HhDd:hhDd)

Phenotypic Ratio 1:1 (wavy hair with dimples: straight hair with dimples)

Therefore, all offspring with have a 50% chance to have either wavy hair or straight hair and 100% of offspring will have dimples

Complete the cross.

Determine the resulting phenotypic ratio for the F1 offspring.

Phenotypic ratio – 9:3:3:1

9 black/short, 3 black/long, 3 brown/short, and 1 brown/long

Objectives for Linked Genes

1. Define linkage group

2. Explain an example of a cross between two linked genes

3. Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

Linked Genes

• Linkage group – two or more genes inherited together because they are found on the same chromosome

• Linked genes do not follow Mendel’s Law of Independent Assortment which would give a typical result of 9:3:3:1 (instead they give a wide variety of ratios)

Common Example of Linked Genes

• For fruit fly Drosophila the gene for body color and wing length are linked b/c they are located on same chromosome

• For fruit fly:

G = grey body

g = black body

L = long wings

l = short wings

1. In a lab we have two parent flies with alleles shown: (linkage shown)

G L g l G L g l

2. These two flies mate and their offspring gets one trait from each parent G L

g l3. Then, the offspring mates with another fly

which is homozygous recessive (grey/long wings mates with black/short wings)

G L g l g l g l

4. Complete a Punnett grid to show the results of the mating

5. Notice the two offspring in bold are in combinations unlike their parents (grey/short wings and black/long wings) therefore they must have formed through crossing over and we call them recombinants

GL Gl gL gl

gl GgLl Ggll ggLl ggll

gl

gl

gl

Objectives for Polygenic Inheritance

1. Define polygenic inheritance

2. Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin color

Polygenic Inheritance

• It is when 2 or more genes influence the expression of one trait

• Since there are 2 or more allelic pairs found at different loci, the number of possible genotypes is greatly increased

*This is why interpreting the human genome is especially difficult since many genes can effect

the same trait

Continuous Variation• When multiple genes

produce a spectrum of resulting phenotypes (this can make it so the genotype is not clear)

• Examples include skin color: multiple genes effect the intensity of pigment in the skin (at least 3 genes affect skin color)

• Other examples of continuous variation include eye color, height, body shape and intelligence

• Note: many of these traits are also affected by the person’s environment (nature vs. nurture) so they are called multifactorial

• Traits that do not vary along a continuum are referred to as discontinuous variation; examples of these traits include your blood group, widow’s peak, attached vs. unattached earlobes