Summary Unit 7 - The control of gene expression

3.7.1 Inheritance
10 March 2022 08:58



The genotype is the genetic constitution of an organism.
The phenotype is the expression of this genetic constitution and its interaction with the
environment.

There may be many alleles of a single gene.
Alleles may be dominant, recessive or codominant.
Recessive - an allele is only expressed if there are no dominant alleles present
Dominant - allele that will always be expressed in the phenotype
Codominant - both alleles are dominant and equally expressed in phenotype
BIBA

In a diploid organism, the alleles at a specific locus may be either homozygous or heterozygous.
Homozygous - a pair of homologous chromosomes carrying two of the same alleles for a gene
Heterozygous - a pair of homologous chromosomes carrying different alleles for the same gene

The use of fully labelled genetic diagrams to interpret, or predict, the results of:
• monohybrid and dihybrid crosses involving dominant, recessive and codominant alleles
Monohybrid - genetic inheritance cross of characteristic determined by one gene
B/b
Dihybrid - genetic inheritance cross of characteristic determined by two genes

1. Cystic Fibrosis is caused by a recessive allele. If two carriers reproduce, what is the likelihood
they will have a child who has it?
Parent genotype = Ff x Ff

gametes F f
F FF Ff
f Ff Ff

25 percent probability

What's the probability of having a girl with cystic fibrosis?

0.5x0.25=0.125

2. Cows can be red/white/roan. Red and white are dominant alleles. If two roan cows reproduce,
what is the chance they will have red offspring?

Parental genotype: CWCR x CWCR

gametes CW CR
CW CWCW CRCW
CR CWCR CRCR

25 percent
Phenotype = CRCR = red

3. Two characteristics of peas are whether they're wrinkled/round and if they're yellow/green.
These characteristics are coded for by two different genes. Round allele is dominant. Yellow allele is
dominant.

Genetics, populations, evolution and ecosystems Page 1

, dominant.
Phenotypes: round, yellow/round, green/wrinkled, yellow/wrinkled, green

If phenotypes of parents are round, yellow and wrinkled, green
First generation
Genotypes: RRYY x rryy
Possible gametes: RY x ry
Offspring genotype : RrYy
Offspring phenotype: round, yellow
Proportion: 100 percent

Second generation
Parent genotype: RrYy x RrYy
Possible Gametes: RY, Ry, ry, rY
RY Ry ry rY
RY RRYY RRYy RrYy RrYY
Round Round Round Round
yellow yellow yellow yellow
Ry RRYy RRyy Rryy RrYy
Round Round Round Round
yellow green green Yellow
ry RrYy Rryy Rryy rrYy
Round Round Round Wrinkled
Yellow green green yellow
rY RrYY RrYy rryY rrYY
Round Round Wrinkled
Yellow Yellow Yellow



9:3:3:1
If two heterozygous parents crossed, always get this ratio
Unless there's crossing over/autosomal linkage

Offspring ratios are not fixed/fertilisation is random B


When homozygous dominant, homozygous recessive, offspring have heterozygous

• crosses involving sex-linkage, autosomal linkage, multiple alleles and epistasis.
Multiple alleles - more than two alleles for a single gene
IBIA
Sex-linkage - gene whose locus is on X chromosome
XaXA
Males more likely to inherit recessive condition, as they only need one copy of the allele on the X
chromosome.
Inherited from mother as they inherit Y chromosome from father.
Autosomal linkage - genes that are located on the same chromosome (not sex chromosome)
AaBb
Epistasis - when one gene modifies expression of a different gene at a different locus
AA

1. Blood group is both codominant and multiple alleles
4 different phenotype : A/B/AB-codominant/O (refers to antigen on red blood cells)





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