Inheritance Flashcards
Zygote
Fertilised egg cell
Allele
Alternative form of a gene
Dominant gene
Always expressed in phenotype
Recessive gene
Only expressed in phenotype when homozygous
Homozygous
Pair of chromosomes that have two of the same alleles of a gene
Heterozygous
Pair of chromosomes that have two different alleles of a gene
Genotype
Combination of alleles for a particular gene
Phenotype
Expression of characteristic due to genotype AND environment
Monohybrid inheritance
Inheritance of a single phenotype affected by one gene
Acronym for Genetic Diagram
First
Inside
Outside
Last
Genetic Diagram Points (5)
(Parent phenotype) Parent genotype Gametes Offspring genotype (in table) Offspring phenotype with proportions (for monohybrid match genotype to phenotypes but with dihybrid give phenotypes in ratio)
Test cross
- Determine if individual with dominant characteristic is homozygous or heterozygous
- Crossbred with homozygous recessive individual (give genotype)
- All offspring dominant = homozygous
- Some offspring recessive = heterozygous
How to prove gene is dominant
Affected parents produce unaffected offspring
How to prove gene is recessive
Unaffected parents produce affected offspring
Codominant gene
Both alleles are equally dominant so both are expressed in phenotype
Multiple Alleles
More than two alternative forms of a gene (alleles) that can occupy the same locus, of which only two can be present in a single organism’s homologous chromosomes
E.g. ABO system of blood groups is controlled by three alleles, only two of which are present in an individual
Sex-linked gene
A gene found only on sex chromosomes, i.e. X chromosomes in humans
Differences between X and Y chromosomes
- X larger compared to Y
- X contains more genes
Suggest why sex-linked diseases are more common in men compared to women
- men only have one X chromosome
- Y chromosome does not have an equivalent non homologous portion as X
- recessive alleles on non homologous portion of X are more frequently expressed
Dihybrid Inheritance
Inheritance of two phenotypes determined by different genes located on different chromosomes
Dihybrid Cross
Mating experiment between two organisms both heterozygous for different traits
Gives theoretical ratio 9:3:3:1
What does dihybird cross prove
- 9:3:3:1 ratio
- pairs of alleles for a trait are sorted independently from one another from generation to generation
- independent assortment
Suggest why there are four possible gametes in a dihybrid cross
- genes for both traits on separate chromosomes
- independent segregation occurs during meiosis where chromosomes arrange themselves randomly along equator
Autosomal Linkage
Two or more genes carried on the same autosome (non-sex chromosome) so are linked
What impact does cross over have on autosomal linkage
Cross over creates new combinations of alleles so there are four possible gametes as oppose to two
What determines if two genes are likely to be separated during cross over
Two alleles are less likely to be separated during cross over if they are closer together on chromosome
Epistasis
When one gene prevents the expression of another gene
Dominant Epistasis
Epistatic gene requires at least one dominant allele to prevent expression of other gene
Recessive Epistasis
Epistatic gene must be homozygous recessive to prevent expression of other gene
Phenotypic ratios when breeding parents that are both heterozygous for two genes
9: 3:3:1 (independent genes/no epistasis)
9: 3:4 (recessive epistasis)
12: 3:1 or 13:3 (dominant epistasis)
Explain why observed ratios are often not the same as expected ratios in a population
- small sample size
- random fusion of gametes
- lethal alleles / selective disadvantage
How do multiple alleles of a gene arise
mutations
Suggest what dashes represent in a genotype
Either dominant or recessive allele
Suggest what assumptions are made when using Hardy-Weinberg equation
- no selection
- random mating
- large population / gene pool
- no mutations
- population is isolated
- (generations do not overlap)
- (no immigration or emigration)
Suggest why there might be a high incidence of a genetic disease in a population
- common ancestor
- genetic isolation / small gene pool
- in breeding
- high probability of mating with individual carrying disease causing allele
Suggest how results of a genetic cross demonstrate independent assortment is involved in production of gametes
- new combinations of alleles
- in offspring but not parents
Explain how evidence from a pedigree diagram shows a condition is recessive
- unaffected parents have a child with condition
- both parents must be carriers
Suggest why sex linked genes are found only on the X chromosome
- X chromosome is larger
- Y lacks equivalent non homologous section where sex-linked genes are found
Suggest why number of people with a fatal genetic disease may actually be lower than predicted by the Hardy-Weinberg equation
Some of those with condition die
Suggest what the Hardy-Weinberg principle predicts
The proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next
Describe how to determine if a trait in an organism is genetic or environmental
- breed in a lab
- observe / measure trait
- if trait is present then must be genetic and not environmental
Explain how you can tell genes are linked
- certain phenotypes are more common when you would expect equal numbers
- alleles controlling more common phenotype are linked
- gametes mainly produce linked combination of alleles
- cross over rarely produces other combinations of alleles
Suggest reasons why ratio of offspring might not be the same as predicted
- epistasis OR - autosomal linkage - no independent segregation - linked alleles inherited in same gamete UNLESS cross over occurs
