genetics + linkages Flashcards
What elements should be included in a genetic diagram or Punnett square? (6)
Parental phenotypes
Parental genotypes
Gamete alleles
Offspring genotypes
Offspring phenotypes
Ratio of offspring phenotypes
Why are observed ratios different from expected ratios? (3)
Random fertilisation of gametes
Small sample size
Offspring ratios arise by chance
How do you determine the possible gametes for a monohybrid cross? (3)
For a parent with genotype AA → all gametes are A
For a parent with genotype aa → all gametes are a
For a parent with genotype Aa → the gametes are A and a
What is the Punnett square for a monohybrid cross between two heterozygous parents (Aa x Aa)? (3)
The gametes for each parent are A and a
Genotypes = AA, Aa, Aa, aa
Phenotypic ratio: 3:1 (dominant to recessive trait)
How do you determine the possible gametes for a dihybrid cross? (7)
Use the FOIL method (First, Outside, Inside, Last) to determine gametes for a parent with two genes
Example: AaBb
First: AB
Outside: Ab
Inside: aB
Last: ab
Possible gametes: AB, Ab, aB, ab
How would a dihybrid cross between two heterozygous parents (AaBb x AaBb) look in a Punnett square? (4)
Gametes for each parent: AB, Ab, aB, ab
16 possible offspring genotypes (4x4 grid)
AABB, AABb, AaBB, AaBb, etc.
Phenotypic ratio for a dihybrid cross is typically 9:3:3:1
What is autosomal linkage? (3)
Refers to genes located on the same autosome (non-sex chromosome)
These genes are likely to be inherited together during meiosis because they are physically close on the same chromosome
Are less likely to be separated by crossing over
How does autosomal linkage affect independent segregation of chromosomes? (3)
Autosome-linked genes tend to stay together during independent segregation of chromosomes in meiosis I
Meaning their alleles are often inherited together
This reduces the genetic variation that would typically occur with independent assortment
How can crossing over affect autosomal linkage? (2)
Crossing over during meiosis can separate linked genes if it occurs between the two loci
This can result in new combinations of alleles being passed to offspring
What is an autosome? (3)
Any chromosome that is not a sex chromosome
Humans have 22 pairs of autosomes
What is sex linkage? (1)
When the locus of the allele that codes for it is on a sex chromosome (X or Y)
What sex chromosomes do males and females have in mammals? (2)
Males = XY | Females = XX
Why are most sex-linked genes X-linked? (2)
The Y chromosome is smaller than the X chromosome
So most sex-linked genes are located only on the X chromosome
Why are males more likely to express recessive X-linked phenotypes? (2)
Males have only one X chromosome, so they only have one allele for X-linked genes
Will express the trait even if the allele is recessive
What are examples of genetic disorders caused by faulty alleles on the X chromosome? (2)
Red-green colour blindness
Haemophilia
What is epistasis? (2)
The interaction between different genes
Where the allele of one gene masks the expression of the alleles of another gene
What are the two types of epistasis? (2)
Dominant epistasis
Recessive epistasis
What is dominant epistasis? (1)
A dominant genotype on one gene inhibits the expression of the other gene
What is the expected phenotypic ratio for dominant epistasis? (3)
12 Epistasis (inhibited): 3 Expressed (dominant): 1 Expressed (recessive)
What is recessive epistasis? (1)
A recessive genotype on one gene inhibits the expression of the other gene
What is the expected phenotypic ratio for recessive epistasis? (1)
9 Expressed (dominant): 3 Expressed (recessive): 4 Epistasis (inhibited)
