Inheritance Flashcards
Phenotype
Expression of genetic constitution / genotype / allele(s);
AND its interaction with the environment;
Dominant allele
Is always expressed / shown
in the phenotype;
Recessive allele
Requires both alleles to be present - homozygous recessive genotype - to be expressed / show in the phenotype.
Genotype
The genetic constitution of an organism.
In other words, all the alleles it contains
e.g. Heterozygous OR homozygous recessive
Define a gene
A sequence of DNA bases that codes for a protein
(AO1)
Define an allele
Different versions of the same gene
These are typically created by random mutations
There may be many _____________ of a single gene.
alleles
In a diploid organism, the alleles at a specific __________ may be either homozygous or heterozygous.
locus
Define locus
a
The location/position of a particular gene on a chromosome
If there the two copies of a gene are the same allele in a genotype, this is known as __________________
homozygous
If there the two copies of a gene are different alleles in a genotype, this is known as __________________
heterozygous
What is the expected phenotypic ratio if your cross heterozygous parents (i.e. they carry the recessive allele) in a monohybrid cross?
3 : 1
Where 1 represents the phenotype from the homozygous recessive genotype
A breeder crossed a black male cat with a black female cat on a number of occasions. The female cat produced 8 black kittens and 4 white kittens.
Explain the evidence that the allele for white fur is recessive.
Parents are heterozygous;
Kittens receive white allele from parents / black cat;
A breeder crossed a black male cat with a black female cat on a number of occasions. The female cat produced 8 black kittens and 4 white kittens.
Predict the likely ratio of colours of kittens born to a cross between this black male and a white female.
1:1
A Bb (Black) x bb (White) cross will produce Bb and bb offspring
In fruit flies, a gene for body colour has a dominant allele for grey body, G, and a recessive allele for black body, g.
Explain how you would determine if the genotype of a grey is homozygous or heterozygous for body colour (2 marks).
- Cross with a homozygous recessive black fly (with the gg genotype)
- If some offspring are black, then must be heterozygous/Gg
OR if all offspring are grey, then must be homozygous/GG
What is the expected phenotypic ratio if your cross heterozygous parents for a codominant phenotype.
1 : 2 : 1
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;
Epistasis;
Lethal genotypes;
Gametes produced by AaBb genotype.
The genes are on different chromosomes.
AB , Ab , aB , ab
Expected phenotypic ratio of offspring if both parents are heterozygous in a dihybrid cross.
i.e. AaBb x AaBb
9 : 3 : 3 : 1
Mutation is one cause of genetic variation in organisms.
Give two other causes of genetic variation.
- Crossing over;
- Independent segregation
(of homologous chromosomes); - Random fusion of gametes
OR random fertilisation;
What process during meiosis 1 creates different combinations of alleles in the gametes?
Independent segregation
Meiosis results in cells that have the haploid number of chromosomes and show genetic variation. Explain how (6 marks).
(Crossing over)
1. Homologous chromosomes pair up / bivalents form;
- Crossing over / chiasmata form;
- Produces new combination of alleles;
(Independent segregation) - Homologous chromosomes separate;
- At random;
- Produces varying combinations of chromosomes / genes / alleles;
- Sister chromatids separated at meiosis II;
Which process during meiosis 1 produces recombinant gametes?
a
crossing over
How could you prove a dominant allele is NOT sex-linked?
Explain why.
CLUE = mums & sons
If the allele is sex-linked, homozygous recessive mothers will always have sons that show the recessive phenotype.
If, however, the sons show a dominant phenotype, there is no way he could have inherited this on the X chromosome from his mother.
The dominant allele must be on another chromosome.
