Top4-Chp5-P99-104

Question 1

It was shown in mice that the homozygote yellow allele was lethal? Explain this.

Answer 1

When P generation of yellow furred mice were crossed, which were both Yy, then the YY died early in development, the Yy was yellow and lived and the yy were non yellow and lived. So the dead YY were ¹/₄, the yellow Yy were ¹/₂ and the nonyellow were yy also ¹/₄. But to see the Yy made up ²/₃ and the yy made up ¹/₃, because the dead YY were never born.

Question 2

Phenotype ratios

Answer 2

Phenotypic ratio; genotypes of parents; genotypes of progeny

3: 1; Aa x Aa; 3/4 A_ : 1/4aa
1: 1; Aa x aa; 1/2Aa:1/2aa

Uniform progeny

AA x AA = All AA

aa x aa = All aa

AA x aa = all Aa

AA x Aa = all A_

Question 3

Genotypic ratios see page 56

Answer 3

Question 4

Mendel’s concept of Dominance

Answer 4

Every individual possesses two alleles and only one is observed in the phenotype

Heterozygote has same phenotype as homozygote dominant
Dominance is not universal

Question 5

Incomplete dominance?

Answer 5

when the heterozygote has a phenotype intermediate between the phenotypes of the two homozygotes.

Ie red and white flowers can produce pink, or purple and white fruits can produce violet colour.

Question 6

Types of dominance: Dominance

Answer 6

Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes

Question 7

Types of dominance: Incomplete dominance

Answer 7

Phenotype of the heterozygote is intermediate (falls within the range) between the phenotypes of the two homozygotes. It does not need to be exactly intermediate. It might be a slightly lighter shade of red or a slightly pink shade of white. It is on a slider.

Question 8

Types of dominance: Codominance

Answer 8

Phenotype of the heterozygote includes the phenotypes of both homozygotes.

Question 9

When a trait exhibits incomplete dominance, a cross between two heterozygotes produces a ___________ phenotypic ratio in the progeny

Answer 9

1:2:1

Question 10

Explain how codominance is shown by the MN blood types.

Answer 10

In MN blood types heterozygotes with genotype L^ML^N exhibit codominance and express both the M and the N antigens (An antigen is any substance that causes your immune system to produce antibodies against it.); they have blood type MN.

Question 11

The type of dominance exhibited by a characteristic frequently depends on the level of the ________ examined.

Answer 11

phenotype

Question 12

Explain how cystic fibrosis works and how the dominance depends on the level of phenotype observed.

Answer 12

Gene responsible for cystic fibrosis is on the long arm of chromosome 7. It encodes a protein called cystic fibrosis transmembrane conductance regulator (CFTR) which acts as a gate in cell membrane and regulates movement of chloride ions in and out of the cell. Cystic fibrosis patients have a mutated, dysfunctional CFTR causing channel to stay closed causing chloride ions to build up in cell, causing formation of thick mucus and produces symptons of the disease. Most people have two copies of normal CFTR allele producing functional CFTR protein. Cystic fibrosis sufferers produce two CFTR mutated alleles and produce defective CFTR protein. Heterozygotes with one normal and one defective CFTR are codominant with both alleles expressed. However, because one functional CFTR is enough to allow normal chloride ion transport then the mutate CFTR in heterozygotes appears to be recessive at the physiological level.

Question 13

Explain what is meant by dominance is frequently “in the eye of the beholder”

Answer 13

meaning that the classification of dominance depends on the level at which the the phenotype is examined, ie cystic fibrosis.

Question 14

Incomplete penetrance means

Answer 14

that the genotype not always produces the expected phenotype. For example in human polydactyly (extra fingers and toes) the trait is usually caused by dominant allele. The allele is dominant yet parents have gene but it does not show eventhough children get it. The gene is therefore not very penetrant.

Question 15

Penetrance is defined as

Answer 15

the percentage of individuals expressing a phenotype defined for by a genotype. Ie if 42 people have polydactyly allele but only 38 are polydactylous then the penetrance would be ³⁸/₄₂ = 0.90 or 90%

Question 16

Expressivity means

Answer 16

the degree to which a trait is expressed

Question 17

A lethal allele causes

Answer 17

death at an early stage of development - often before birth - and so some genotypes may not appear among the progeny, modifying the ratio of progeny resulting from a cross.

Question 18

Explain the peculiar inheritance effects that lethal alleles produce.

Answer 18

• The lethal alleles is dominant in heterozygotes while the lethal effect is recessive
• Truly dominant lethal alleles cannot be transmitted unless they are expressed after the onset of reproduction as in Huntington Disease.

In the case of mice below the lethal allele is recessive because it causes death only in homozygotes, yet the yellow colour is dominant.

Question 19

Multiple alleles are? Explain how this works in organism populations and individuals and how this works for Mallard ducks

Answer 19

Some loci have more than two alleles present.

But note: a group of organisms may have more than two alleles but the genotype of each individual diploid organism still consists of only two alleles.

In Mallard ducks there are three alleles and dominance is shown M^r > M > m^d. An M^Rm^d cross with Mm^d will produce ¹/₄ M^RM, ¹/₄ M^Rm^d, ¹/₄Mm^d and ¹/₄m^dm^d. And the phenotype will be 1/2 M^R_ , 1/4 M_ and 1/4m^d_

Question 20

Give the formula for the number of genotypes possible

Answer 20

[n(n + 1)]/2 where n equals the number of different alleles at a locus