Punnet Squares, Linked Genes and Pedigrees

Predicting offspring genotype

• If we know the genotype of the parents, we can predict the genotype of the offspring, using test crosses (punnett squares)

• Alternatively, parental genotype may be able to be determined if the offspring genotype is known

ExampleTracey

Phenotypes:

Blood group B

Normal skin pigment

Genotypes:

IBi

?

John

Phenotypes:

Blood group B

Normal skin pigment

Genotypes:

IBi

?

Samson

Phenotypes:

Blood group O

Albino

Genotypes:

?

Working out the possibilities

• Step 1: work out the possible genes within the sperm and egg cells

• Step 2: perform monohybrid cross

PUNNETT SQUARE: BLOOD TYPES

IB i

IB

i

What is the probability of heterozygotes producing different phenotypes?

PUNNETT SQUARE: HETEROZYGOTES

A a

A

a

Genotype ratio:

Phenotype ratio:

Complete the following:

• Punnett Squares made easy worksheet

What if we want to work with combinations?

• We want to find out all the combinations of skin pigment and blood types that Tracey and John’s children could have.

• Step 1: work out all the genotype possibilities within the egg and sperm cells

• Step 2: perform dihybrid test cross

Dihybrid crossIBA IBa iA ia

IBA IBIBAA IBIBAa IBiAA IBiAa

IBa IBIBAa IBIBaa IBiAa IBiaa

iA IBiAA IBiAa iiAA iiAa

ia IBiAa IBiaa iiAa iiaa

Genotype ratio:

Phenotype ratio:

Linked genes

• Genes are said to be “linked” when their loci are found on the same chromosome.

• It means that those alleles are usually inherited together… but not always!!

http://www.schenectady.k12.ny.us/putman/biology/data/images/inheritance/5vtxrn.gif

The offspring of this couple gets one of each parental chromosome. The genes are intact, so the alleles that are inherited are of the “parental type”

Recombination

• Remember we said that in meiosis, genetic material can move from one chromosome to another?

• This is called recombination. It results in offspring having chromosomes that are not identical to parental chromosomes.

Recombination

http://www.sayhelloandsmile.com/parta/paper1knowledge/2_diseasecausationdiagnostic/2d_genetics/2d3_e.jpg

You can see that the probability of genes that are close together (a and b) being separated during crossing over is less than that of genes that are further apart (a and c)

Recombination• In the example below, the mother is

heterozygous at both loci, while the father is homozygous at both.

G G

a a

G g

a A

Parental types:

Ga, gA (the codes found on the parental chromosomes)

If crossing over occurs in maternal meiosis, there is no change, as the alleles are the same on each. HOWEVER, if there is crossing over in the paternal chromosomes, and the G/g alleles swap places, there will be RECOMBINANT offspring with chromosomes with the following allelic combinations:

ga or GA

Detecting linkage

• If you know the number of recombinant offspring (those without the parental type chromosomes), you can work out how far apart the genes are from each other.

• This is the equation to use:

100 x number of recombinant offspring

total number of offspring

Detecting linkage example

Paternal genotype

Parental type Recombinant

Ga gA GA ga

Ga GaGa GagA GaGA gaGa

44% 44% 6% 6%

Maternal genotype

So, using our formula before (although, we have percentages, it’s much easier!):

Distance between loci = 100x number of recombinant (6+6=12) = 12 map units

Total number of offspring (44+44+6=100)

Detecting linkage problemParent 1 Parent 2

A A

Q q

A

Q q

a

Parent 1

(all parental)

Parent 2

Parental

Recombinant

AQ Aq

AQ AAQQ AAQq

aq AaQq Aaqq

Aq AAQq AAqq

aQ AaQQ AaQq

Number of parental type genotypes:_______

Number of parental type offspring: 212

Number of recombinant genotypes: _________

Number of recombinant offspring: 156

Distance between loci:

Pedigrees

• Family tree style diagrams that show the presence/absence of particular disorders within a family.

• Has a distinct set of symbols

Pedigree Key

Carrier (heterozygote, in recessive disorders)

Autosomal dominant patternCharacteristics of pattern:

-Both males and female affected

-All affected individuals MUST have an affected parent

-Once the trait disappears from a branch of the pedigree, it does not reappear

-In a large sample, approx. the same number of each sex will be affected

Autosome: any chromosome that is not a sex chromosome

Autosomal recessive pattern

http://www.migeneticsconnection.org/genomics/family%20history/AutosomalRecessiveOnly1.gif

Characteristics of pattern:

-Both males and females can be affected equally

-Two unaffected parents can have an affected child (why… and what symbol could be useful here?)

-All children of two people with the disorder MUST also be affected

-The trait may disappear from a branch of the pedigree, but reappear later on

X linked dominant patternCharacteristics of pattern:

-Male with trait passes it on to all of his daughters, and none of his sons

-Female with trait may pass it to both daughters and sons

-Every affected person has at least one parent with the trait

-If the trait disappears from a branch, it does not reappear

-More affected females than males

http://www.biolab.co.in/uploads/biolab/5dafedf3f5770fc5aa9654ea05c5c0a3.jpg

X linked recessive pattern

http://journal.hsmc.org/ijnidd/uploadedImages/figure4.GIF

Characteristics of pattern:

-All sons of an affected female are affected

-All daughters of a father with the trait are carriers

-None of the sons of an affected father and unaffected mother will show the trait UNLESS their mother is a carrier

-All children of two affected individuals will also be affected

-More males than females show the trait

In groups:

• You will be given one of the pedigree patterns. – Provide explanations for EACH of the

characteristics of the pattern– Find out about at least two disorders with this

pattern of inheritance