Human Genetics Flashcards
What is an allele?
The type of a particular characteristic that is determined by a gene.
What does heterozygous mean?
Individuals with two different alleles.
What does homozygous mean?
Individuals with two copies of the same allele.
What is a genotype?
The genetic constitution, summing all of the alleles.
What is a phenotype?
The observed form of a characteristic in an organism.
What is the exome?
The DNA sequence that is complementary to all the mRNA produced from the genome.
What is the principle of segregation?
- Characteristics are determined by genes, existing in pairs in each organism.
- Each gamete contains only one of each pair- which is entirely random- and therefore these factors segregate.
- The Union of male and female gametes is a random process that reunites pairs of genes.
What are the two observations that make up the basis of genetics?
- One form of life always gives rise to other individuals of the same type.
- While there is individual variation within a species, that variation is not random since individuals have a tendency to resemble their ancestors.
What is a null mutation?
A mutation that results in the loss of gene function.
What is a hypomorph mutation?
A mutation that results in partial gene function.
Examples of gain of function mutations
Dominant active, dominant negative, Neomorph, hypermorph and antimorph
What is a Neomorph mutation?
A gain of function mutation that results in the introduction of a new function.
What is a hypermorph mutation?
A gain of function mutation that results in more of the original gene function.
What is an antimorph mutation?
A gain of function mutation where the function gained inhibits the wild type gene function.
What is a silent mutation?
A mutation that does not affect gene function.
What is an example of a dominant active mutation?
Mutation of the Ras gene, which causes a GTPase to become constitutive.
Three gene sites sensitive to null mutations
- Mutation in the conserved region of a promoter sequence, inhibiting transcription.
- Mutation in an enzyme’s active site, changing the structure/function of the enzyme.
- Mutation in an intron, meaning that the intron is not removed during splicing, and incorrect proteins are produced.
Example of a gene site sensitive to a hypomorph mutation
Mutation at the edge of an enzyme’s active site, resulting in a slight change to the enzyme structure.
Example of a gene site sensitive to silent mutations
Mutation anywhere other than the active site of an enzyme.
Define Haplosufficient
A gene which has enough function in a heterozygote to be functionally wild type.
Define haploinsufficient
A gene which does NOT HAVE ENOUGH function in a heterozygote to be functionally wild type.
- one copy of the wild type allele is not enough
- cannot function if one allele is mutant
Which types of mutant alleles are normally recessive?
Alleles that have resulted from either null mutations or hypomorph mutations.
Which types of mutant alleles are normally dominant?
Alleles that resulted from gain of function mutations.
- dominant active, dominant negative, Neomorph, hypermorph, antimorph
Define dominant allele
The trait that is shown in the heterozygote.
Define recessive
The trait that is hidden in a heterozygote and is only visible in a double recessive homozygote.
How did Mendel’s peas show complete dominance?
In all of the crosses that Mendel did, the heterozygote had the same phenotype as the homozygous dominant.
An example of a gene that is not completely dominant/recessive
The MCR1 alleles for red hair are not completely recessive as some heterozygotes have red hair.
A snapdragon can appear pink as a result of a cross between a red plant and a white plant as the alleles are codominant, as the red allele isn’t completely dominant and the white allele isn’t completely recessive.
What is pedigree analysis?
Inference of the random human mating that must of occurred in previous generations, used to follow genetic inheritance.
What are the features of dominant inheritance?
Dominant traits are present in all generations.
Two individuals with a dominant trait can still produce homozygous unaffected progeny.
What are the features of recessive inheritance?
Recessive traits can skip a generation- can get affected individuals from unaffected parents.
Shows individuals who must have been carriers of the recessive allele in pedigree diagrams.
Recessive traits can only be seen if both alleles are mutant and not wild type.
If there is mating between two affected parents, all progeny are affected.
What is a wild type allele?
The most frequent allele of a gene.
What is a mutant allele?
All alleles other than the wild type allele for a particular gene.
Define polymorphic and give an example of a polymorphic gene.
Several alleles of a gene are all frequent, even if one is more rare than another, and no allele is chosen as wild type, e.g. blood group gene.
What is a heteromorphic pair? What is an example of a heteromorphic pair?
A pair of chromosomes that are not homologous and do not look alike, are in different forms.
An example are the X/Y chromosomes in humans.
What are heteromorphic pairs associated with and where are they found?
The heteromorphic pair are the sex chromosomes. In one sex, there is the heteromorphic pair and in the other sex there are two copies of one member of the heteromorphic pair.
What are autosomes?
Chromosomes which are identical in both sexes, e.g. Chromosomes 1-22 in humans.
How do the X and Y chromosomes behave in meiosis?
As homologues.
The Y chromosome from a father must go to a son and the X to a daughter.
The X chromosome in any male must have come from their mother.
Half the population in any generation have a Y chromosome.
Define Nondisjunction.
Exceptionary behaviour of chromosomes, e.g. In Down’s syndrome
What is a lethal allele?
Alleles that cause death when present in a homozygote.
What is phenylketonuria?
It is a disease caused by a rare recessive allele. The allele means that the phenylalanine hydroxylase enzyme cannot convert Phe to Tyr.
How is phenylketonuria diagnosed and treated?
It is diagnosed at birth using a Guthrie test which uses a PKU antibody to check for the enzyme in a baby.
It is treated by a low protein diet, which limits Phe build up in the cerebrospinal fluid.
What is independent assortment?
Alleles of a particular gene are inherited independently to the alleles of another gene
Explain the 9:3:3:1 ratio
Mendel looked at two genes, when crossed it produced a doubly heterozygous F1 generation. Allowing this F1 to self-fertilise produces progeny that are either dominant for both characteristics, dominant for one or the other or recessive for both, in the ratio 9:3:3:1.
What is the trihybrid ratio?
27:9:9:9:3:3:3:1
What are the properties of the chi squared test?
- appropriate if p doesn’t equal q
- appropriate if there are more than two outcomes
- not appropriate if the number of expected is small
What is epistasis?
The interaction of different genes, which causes one gene to mask the phenotype of another gene.
What is linkage?
When genes are on the same chromosome, there is a tendency for alleles of those genes to be inherited together.
