Chapter 17 inherited change notes from AQA A Level Biology (2nd edition). Authors: Glenn Toole, Susan Toole Publisher: Oxford University Press (including Nelson Thornes) with specification reference and exam questions at the end.
17 Inherited change
17.1 Studying inheritance
Genotype and phenotype
Genotype: genetic make-up of an organism
o Describes all alleles and determines the limits w/in characteristics of an
individual may vary (eg someone could grow so tall, but this is inhibited
by environment like diet)
Phenotype: observable/biochemical characteristics
o Result of interaction btw expression of genotype
and environment
Genes and alleles
Gene: length of DNA, sequences of nucleotide bases, that
code for a particular polypeptide
o Exist in 2 or more forms, alleles
o Locus: position of a gene on a particular DNA
Allele: alternate form of a gene (eg gene: hair colour,
alleles: brown/blonde hair)
o Only 1 allele of a gene can be at a particular
place (loci) on a chromosome
o Homologous chromosomes: pair of
chromosomes in diploid organisms ∴ 2 loci,
carrying each allele of a gene
o Homozygous: same allele | heterozygous: diff
o Dominant: allele of heterozygote that is
expressed in the phenotype | recessive: not
expressed (only in homozygous state)
o Codominance: 2 alleles contribute to phenotype
(eg mixture of colour/ AB blood group)
o Multiple alleles: more than 2 (however only 2
out of 3/more can be present on homologous chromosomes)
17.2 Monohybrid inheritance
Representing genetic crosses
Inheritance of pod colour in peas
Monohybrid inheritance: inheritance of a single gene
Eg Gregor Mendel’s pea pods, come in green/yellow
o Pure breeding: strains bred for a character (eg green pods are bread repeatedly w/ each other to produce
green pods) ∴ organisms are homozygous for that gene)
If pure green is crossed w/ pure yellow, all offspring ae F 1 (first filial) generation and produce green bc
green is dominant to yellow
, If 2 F1 intercross, offspring (F2 gen) will produce ¾ green and ¼ yellow
Law of segregation: in diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each
pair of alleles can be present in a single gamete
17.3 Probability and genetic crosses
Ratios
Ratio: measure of relative size of 2 classes expressed as a proportion
Why actual and predicted results are usually diff
Rarely did Gregor actually get 3:1 for F2, this is due to statistical error (is it by chance that certain gametes fuse with
each other)
Larger sample give a more representable result and are closer to the predicted
17.4 Dihybrid inheritance
2 diff genes on diff chromosomes
inherited
Gregor Mendel:
o Seed shape
(round/wrinkled) and seed
colour (yellow/green)
o Crossed pure breeding
types: round yellow (pure
dominant) and wrinkled
green (pure recessive),
producing all pure dominant (round yellow)
o F1 produce 4 diff gametes
Chromosomes arrange themselves randomly in meiosis: any 1 of the 2
alleles for shape and can combine w/ any 1 of the 2 for colour
Random fertilisation: any 1 of the 4 gametes produced from 1 parent can combine w/ any of 4
gametes from other parent
o F2 give 9:3:3:1 ratio
Law of independent assoetment: each member of a pair of alllels may combine
randomly w/ either/another pair
17.5 Codominance and multiple alleles
Codominance: both alleles expressed in phenotype
Multiple alleles: >2 alleles, but only 2 at loci of homologous chromosomes
Eg: in cattle, allele codes for red hair pigment and other is faulty and codes for lack of
pigment – each allele are dominant
o If homozygous pigment allele, red colour: no pigment allele, white: if heterozygous roan colour
Notation is CX in this case x is R/W
Multiple alleles
Eg: human blood ABO groups for gene I (immunoglobin gene), lead to diff
antigens on cell surface membrane of rbc
o Allele IA IB IO produce antigens A, B, and O: A and B are codominant to
each other, O recessive to both
o Interesting crosses: crosses AB and O only produce A and B offspring
o Heterozygous A and B (w/ O) have offspring w/ any four
17.6 Sex-linkage
23 chromosome pairs, 22 homologous partners identical in male/female, 1 sex chromosome
pair (female XX, male XY smaller)
Sex linkage haemophilia
Any gene that is carried on sex hormones is sex linked
X chromosome is much longer ∴ most of it doesn’t have a homologous equivalent in the Y
chromosome
o ∴ characteristics controlled by recessive allele on the non-
homologous X part appear more frequently bc they don’t have a
homologous Y section that could have the dominant allele and ∴
prevent recessive allele being expressed
X linked genetic disorder: caused by an defective
gene on X chromosome
o Eg haemophilia (blood clots slowly and
persistent internal bleeding), there has
been some selective removal of gene from
population ∴ relatively rare occurrence
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