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Chapter 8 – Inheritance and Selection
What are meant by the terms genotype and phenotype?

Genotype is the genetic make-up of an organism, which describes all the alleles of the genes
it posses
...

Phenotype is the observable characteristics of an organism, resulting from the interactions
between genotype and the environment
...
Different forms of the same gene are known as alleles
...

Chromosomes occur in homologous pairs, each containing the allele for the same gene in
the same locus
...


A dominant allele is one which only needs to be present in one homologous chromosome to
express itself in the phenotype of an organism
...

A co-dominant allele occurs where the presence of two different alleles results in a
phenotype which is a blend of the two, for example in some flowers the presence of a red
and white petal allele produces pink petals
...


How are genetic crosses represented?

Genetic crosses can be represented using a Punnett square, with shorthand letters used to
represent the different alleles
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If an organism is pure-bred for an allele then the homologous chromosomes it posses will be
homozygous for the desired chromosome
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How is sex determined genetically? What is sex linkage?

All females have 2 X chromosomes, and therefore produce 2 X gametes
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Where a gene is carried on either the X or Y chromosome, it is said to be sex linked
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This makes recessive alleles in this extended portion of the X chromosome more likely to
occur in males, as the dominant allele to mask its effect is not present on the Y
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How do multiple alleles affect inheritance? How are blood groups in humans inherited?

Some genes have more than 2 alleles, for example human blood groups:




IA – antigen A
IB – antigen B
IO – no antigen

A and B are co-dominant, whilst O is recessive to both alleles, the possible combinations are
shown below:

Combination
IA IA
IA IO
IB IB
IB IO
IA IB
IO IO

Phenotype
A
B
AB
O

What is meant by the terms gene pool and allelic frequency?

The gene pool is all the different alleles of all the genes present in a population of a species
...
For
example with the blood groups above, whilst IOIO is recessive, as the IO allele is most
common, the O phenotype is the most common
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It relies on 5 assumptions:






There are no mutations
The population is isolated, so there is no influx or loss of alleles
No genetic selection, all alleles are equally likely to be passed onto the next
generation
The population is large
Mating is random

How can the Hardy-Weinberg principle be used to calculate allele, genotype and phenotype frequencies?

Frequency of the dominant allele = p
Frequency of the dominant allele = q
p+q=1

As there are only 2 alleles, the total frequencies must be 100%

There are only 4 possible combinations, which can be expressed:
𝑝2 + 2𝑝𝑞 + 𝑞 2 = 1
For example, if we know that the frequency of a recessive sufferer of a disease is 1/25,000
...

Therefore 𝑞 2 = √1/25,000 = 𝑞 = 0
...
00063 = p = 0
...
00063 and p = 0
...
00063 x 0
...
0125
To work out the number of individuals who are heterozygous, simply times the frequency by
the population size – 0
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These successful individuals are more likely to survive and therefore have offspring who
inherit the advantageous allele
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What is selection? What environmental pressures exert selection pressure?

Selection is the process whereby organisms which are better adapted to their environment
survive and breed, whilst those who are not as well adapted are less likely to do so
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Selection
changes the shape of this curve by increasing the frequency of certain
alleles which produce advantageous phenotypes
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This means that individuals towards this end are more likely to
reproduce and pass on the allele for this
phenotype, meaning the curve shifts in one
direction
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They will then pass on this allele for longer fur length whilst other organisms
will not, meaning the frequency of the longer fur length allele increases in the population
and the curve shifts towards the right
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For example in baby birth weight in humans; too small a child is unlikely to survive in the
natural environment due to heat losses, whilst too large a child is likely to have
complications in birth
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