13.6 Genetics and Inheritance Flashcards
Define gene
A section of DNA that codes for a specific protein.
Define genotype
Full genetic constitution/make-up of an organism and all the alleles it possesses.
Define gene pool
All the alleles within an interbreeding population at a specific time.
Define population
All individuals of the same species occupying the same area at the same time.
Define phenotype
Expression of its genetic constitution/make-up and its interaction with the environment (physical appearance).
Define allele
Different versions of the same gene.
Define locus
Different alleles for the same gene are found at the same position on the homologous chromosome.
Define homozygous
Two copies of a gene are the same allele.
Define heterozygous
Two copies of a gene are different alleles.
Define dominant allele
Always expressed in phenotype.
Define recessive allele
Only expressed in phenotype if there are two copies// if genotype is homozygous.
Define codominant alleles
Equally expressed within phenotype
Define diploid
Containing two copies of a gene
Define haploid
Containing one copy of a gene
Define allelic frequency
Number of times an allele occurs within a gene pool.
Types of inheritance
- Monohybrid
- Codominance
- Dihybrid
- Epistasis
- Autosomal linkage
What does the Hardy-Weinberg principle predict?
Predicts the ‘allelic frequencies (of a particular gene) from one generation to the next will remain constant’.
What does the Hardy-Weinberg principle rely on for it to be correct?
It relies on the fact there will be:
- NO migration
- NO gene mutation
- NO selection for or against a particular allele
- a large population
- random mating in population so all individuals have an equal chance of mating
Hardy-Weinberg equations
p + q = 1
P^2 +2Pq + q^2 = 1
Define monohybrid inheritance
The inheritance of a single gene, which determines a single characteristic.
An example would be where the phenotype is considered by two alleles, one of which is dominant and the other is recessive.
Explain codominance.
Where alleles do not show a standard dominant/recessive relationship.
Small superscript letters used for alleles paired with letter ‘C’
Explain multiple alleles- monohybrid inheritance.
For many genes, there are several alleles in the population (hair colour)
Show a dominance hierarchy
There was 850 fruit flies in a population.
510 fruit flies had the genotype W(H)W(N)
255 fruit flies had the genotype W(N)W(V)
85 fruit flies had the genotype W(V)W(V)
CALCULATE ALLELE FREQUENCY OF ALLELE W(V)
1700 alleles in frequency
2x85=170
1x255=255
255+170=425
424/1700
0.25
950 flies in population
frequency of W(V) allele. was 0.2
calculate the number of expected individuals with the genotype W(N)W(V)
2X0.2X0.8=0.32
0.32X950=
304
Rules for dominant characteristics
- Affected offspring must have at least one affected parent.
- Unaffected parents must only have unaffected offspring.
- If both parents are affected and have an unaffected offspring, both parents must be heterozygous.
Rules for recessive characteristics
Unaffected parents can have an affected offspring if they are heterozygous
Explain sex-linked characteristics- monohybrid inheritance.
One that is controlled by a gene located on one of the sex chromosomes. Gene is on X chromosome.
Male chromosomes
XY
Female chromosomes
XX
Why is it easier for a genetic disease to occur in men?
For sex-linked genes, a female would have to have 2 copies of a recessive allele for it to appear in the phenotype, whereas males only need one.
Sex link rules to prove a characteristic is not sex-linked.
DADS AND DAUGHTERS (R)
- If a daughter is affected (homo R) and her father is not.
- And it is a recessive disease
It is not sex-linked.
MUMS AND SONS (D)
- If a son is affected and his mum is not (homo R).
- And it is a dominant disease
It is not sex-linked.
Define dihybrid inheritance
Involves a phenotype that is inherited as the result of two different genes, thus two characteristics. e.g round yellow
The two genes either can be on different chromosomes or linked on the same chromosome.
The ratio for dihybrid inheritance with homozygous parents
9:3:3:1
How to find possible gametes for dihybrid inheritance
AaBb
1st and 3rd, 1st and 4th
2nd and 3rd, 2nd and 4th
= AB, Ab, aB, ab
Define autosomal chromosome
Chromosomes that are not directly involved in determining the sex of an organism.
Explain autosomal linkage- dihybrid inheritance.
Chromosomes not directly involved in determining sex of organism inherited together.
Explain the crossing-over process
Non sister chromatids on homologous pairs cross over to form a bivalent
Then forms a chiasma
Exchange of alleles to produce random recombinant offspring which display new combination of alleles
What happens to the expected ratio when two genes are autosomal linked
Does not follow the expected phenotypic ratio
Define epistasis
The allele of one gene affects or masks the expression of another in the phenotype;
KP: In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
Suggest two reasons why.
- Small sample size;
- Fusion/fertilisation of gametes is random;
- Linked Genes; Sex-linkage / crossing over;
- Epistasis;
- Lethal genotypes;
KP: What does Hardy Weinberg’s equation predict
- The frequency/proportion of alleles (of a particular gene);
- Will stay constant from one generation to the next/over generations / no genetic change over time;
- Providing no mutation/no selection/population large/population genetically isolated/mating at random/no migration;
KP: Expected offspring phenotype ratios from heterozygous parents:
1. Monohybrid
2. Dihybrid
3. Epistasis
4. Autosomal linkage
Dominant : recessive
- 3:1
- 9:3:3:1
- 9:4:3 or 15:1 or 9:7
- 3:1 (no x over) (no other pattern other than 4 phenotypes with recombination of alleles)
KP: How do multiple alleles of a gene arise?
- mutations;
- which are different / at different positions in the gene;
