Lecture 19 (Extensions of Mendelian genetics)

Question 1

Why is the phenotype for SS and Ss the same?

Answer 1

This is because S is dominant over s

Question 2

What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross?

Answer 2

The probability is 1/16

Question 3

What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross? - How can we achieve ss?

Answer 3

P of parent 1 supplying s = 1/2
P of parent 2 supplying s = 1/2

1/2 x 1/2 = 1/4

Question 4

What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross? - How can we achieve yy?

Answer 4

P of parent 1 supplying y = 1/2
P of parent 2 supplying y = 1/2

1/2 x 1/2 = 1/4

Question 5

What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross? - what is the probability of ss and yy occurring?

Answer 5

What is the probability of both of these events occurring …

1/4 x 1/4 = 1/16

Question 6

Product rule

Answer 6

Use the product rule for independent events e.g. chance of s allele from parent one and chance of s allele from parent 2 are totally independent events and therefore can be multiplied together

Question 7

AaBBCc x AaBbcc = AaBbcc

How can we achieve Aa?

Answer 7

P of parent 1 supplying A= 1/2
P of parent 2 supplying a = 1/2
1/2 x 1/2 = 1/4

P of parent 1 supplying a= 1/2
P of parent 2 supplying A = 1/2
1/2 x 1/2 = 1/4

1/4+1/4 = 1/2 (sum rule)

Question 8

AaBBCc x AaBbcc = AaBbcc

How can we achieve Bb?

Answer 8

P of parent 1 supplying B= 1
P of parent 2 supplying b = 1/2
1x1/2 = 1/2

Question 9

AaBBCc x AaBbcc = AaBbcc

How can we achieve cc?

Answer 9

P of parent 1 supplying c= 1/2
P of parent 2 supplying c = 1
1x1/2 = 1/2

Question 10

AaBBCc x AaBbcc = AaBbcc

What is the probability of the offspring AaBbcc?

Answer 10

(1/4+1/4) x 1/2 x 1/2 = 1/8

Question 11

Sum rule

Answer 11

The probability that one or the other of two mutually exclusive events will occur is the sum of their individual probabilities. The rule that states that the probability of the occurrence of mutually exclusive events is the sum of the probabilities of the individual events. (for examples where it is a or b)

Question 12

Multiple alleles

Answer 12

Three or more alternative forms of a gene (alleles) that can occupy the same locus. However, only two of the alleles can be present in a single organism.

Question 13

Polymorphic

Answer 13

A gene is said to be polymorphic if more than one allele occupies that gene’s locus within a population.

An individual can only have two alleles, one on each homologous chromosome however in a population there can be a bunch of different alleles

Question 14

Incomplete dominance

Answer 14

Occurs when one allele is not completely dominant over the other. The heterozygote produced shows a blend of the two alleles. (you have an intermediate)

e.g. a red and a white flower show incomplete dominance when they produce a pink flower (shown in a heterozygote)

Question 15

Incomplete dominance …When f1 has an intermediate phenotype …

Answer 15

When f1 has an intermediate phenotype, it suggests blending inheritance.

However, in the F2 generation (when two organisms with intermediate phenotypes mate), it rejects the hypothesis of blending inheritance and instead supports particulate inheritance

Question 16

Co-dominance

Answer 16

This is where both phenotypes exist side by side in an organism (both alleles are simultaneously expressed in the heterozygote) (its like superimposing two different phenotypes on each other)

Both parental phenotypes are present in F1

Question 17

Co-dominance in blood

Answer 17

Blood type is also an example of multiple alleles - the ABO system of blood groups is controlled by three alleles, only two of which are present in an individual.

AB blood type is an example of co-dominance as the A is inherited from one parent and the B is inherited from the other parent and are expressed simultaneously

Question 18

Polygenic traits

Answer 18

A polygenic trait is one whose phenotype is influenced by more than one gene. Traits that display a continuous distribution, such as height or skin colour, are polygenic.

Phenotype is controlled by many genes that have an additive effect. Characters appear continuous or quantitive i.e. non-discrete steps in range of phenotypes

Controlled by groups of genes. Polygenic traits do not exhibit complete dominance as do Mendelian traits, but exhibit incomplete dominance.

Question 19

Additive effect of genes

Answer 19

Means that genes are almost working together to create a trait

Additive genetic effects occur when two or more genes source a single contribution to the final phenotype, or when alleles of a single gene (in heterozygotes) combine so that their combined effects equal the sum of their individual effects.

Question 20

Polygenic trait in a population

Answer 20

Normal’ distribution typical of polygenic trait in a population

The more genes involved, the higher the number of phenotypic classes

Question 21

The environment on phenotype

Answer 21

Environment (nature and nurture) also affects phenotype e.g. nutrition and height/weight; life expectancy and disease

The environment smooths difference among phenotypes