Topic 7A: Genetics Flashcards
Gene =
A sequence of bases on a DNA molecule that codes for a protein which results in a characteristic
Allele =
A different version of a gene.
Humans carry 1 from each parent
Genotype =
The genetic constitution of an organism (Bb or bb)
Phenotype =
The expression of the genetic constitution and its interactions with the environment (characteristics)
Dominant =
An allele whose characteristics appears in the phenotype even when there’s only one copy
Recessive =
An allele whose characteristics only appears in the phenotype if two copies are present
Codominant =
Alleles that are both expressed in the phenotype - neither is recessive
Locus =
The fixed position of a gene on a chromosome
Homozygote =
An organism that carries two copies of the same allele
Heterozygote =
An organism that carries two different alleles
Carrier =
A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
How are humans diploid organisms
two sets of chromosomes - 2 alleles of each gene
Genetic diagrams used to
predict the genotype and phenotype of the offspring if parents bred
Monohybrid inheritance =
the inheritance of a characteristic controlled by a single gene
Monohybrid crosses show
likelihood of different alleles of that gene being inherited by offspring
Punnett square =
another way of showing a genetic diagram to predict genotype and phenotype of offspring
Dihybrid crosses look at
how two different genes are inherited at the same time
Phenotype ration in F2: Monohybrid
3:1 (D:d)
Phenotype ration in F2: Dihybrid
9:3:3:1 (DD:Dd:dD:dd)
Phenotype ration in F2: Codominant
1:2:1 (homozygous 1 allele: heterozygous allele: homozygous other allele)
Female sex chromosomes
XX
Male sex chromosomes
XY
Sex linked characteristics =
allele that codes for the characteristic is found on the sex chromosome
X linked genes =
genes that are only carried on the X chromosome
Why are most genes on the sex chromosomes X linked genes
Y chromosomes smaller than X chromosomes so carries fewer genes
Why do males only have one allele for sex linked genes
only 1 X chromosome
Impact of males only having 1 sex linked gene
they only have 1 copy so express the allele even if recessive so more likely show recessive phenotypes for sex linked genes
Genetic disorders caused by
faulty alleles on sex chromosomes
Colourblindness =
sex linked disorder caused by faulty allele carried on the X chromosome
Autosome =
any chromosomes that isn’t sex linked
Why are autosomes genes said to be linked
on same autosome, together during independent segregation in meiosis 1 and alleles passed on to offspring together
When wont autosomes be linked
if the go through ‘crossing over’ in meiosis 1 before segregation
The closer together two genes are on autosome =
more closely linked because crossing over is less likely to split them up
Impact on phenotype ratio if two genes are autosomally linked
more like monohybrid cross ratio than dihybrid because two autosomally linked alleles inherited together - higher proportional offspring with parents geno and phenotypes
Importance of predicted phenotypic ratio
if different from the predicted we can identify autosomal linkage and epistasis
Epistasis =
Allele of one gene masks the expression of alleles of other genes
Epistasis example: widows peak and baldness
Doesn’t matter if you have the allele for widows peak if you have the baldness allele because no hair
