Top4-Chp5-P99-104 Flashcards
It was shown in mice that the homozygote yellow allele was lethal? Explain this.
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.
Phenotype ratios
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_
Genotypic ratios see page 56
Mendel’s concept of Dominance
Every individual possesses two alleles and only one is observed in the phenotype
Heterozygote has same phenotype as homozygote dominant
Dominance is not universal
Incomplete dominance?
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.
Types of dominance: Dominance
Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes
Types of dominance: Incomplete dominance
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.
Types of dominance: Codominance
Phenotype of the heterozygote includes the phenotypes of both homozygotes.
When a trait exhibits incomplete dominance, a cross between two heterozygotes produces a ___________ phenotypic ratio in the progeny
1:2:1
Explain how codominance is shown by the MN blood types.
In MN blood types heterozygotes with genotype LMLN 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.
The type of dominance exhibited by a characteristic frequently depends on the level of the ________ examined.
phenotype
Explain how cystic fibrosis works and how the dominance depends on the level of phenotype observed.
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.
Explain what is meant by dominance is frequently “in the eye of the beholder”
meaning that the classification of dominance depends on the level at which the the phenotype is examined, ie cystic fibrosis.
Incomplete penetrance means
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.
Penetrance is defined as
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%
Expressivity means
the degree to which a trait is expressed
A lethal allele causes
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.
Explain the peculiar inheritance effects that lethal alleles produce.
- 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.
Multiple alleles are? Explain how this works in organism populations and individuals and how this works for Mallard ducks
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 Mr > M > md. An MRmd cross with Mmd will produce 1/4 MRM, 1/4 MRmd, 1/4Mmd and 1/4mdmd. And the phenotype will be 1/2 MR_ , 1/4 M_ and 1/4md_
Give the formula for the number of genotypes possible
[n(n + 1)]/2 where n equals the number of different alleles at a locus
