Biology notes that will help you get an A*
Notes have been made to cater to the AQA Biology Mark Scheme
Contains everything on the spec for Genetics and Populations:
- Inheritance
- Linkage
- Epistasis
- Chi-Squared Test
- Hardy-Weinberg
- Variation and Selection
- Speciation and Genetic D...
Inheritance
Inheritance
Gene - a sequence of bases on a DNA that codes for a protein which results in a characteristic
:
Allele - a different version of a gene.
Genotype - genetic constitution of an organism,
Phenotype - expression of the genetic constitution and its interaction with the environment.
Dominant - an allele whose characteristic appears in the phenotype when there’s only one copy.
-
Recessive - allele whose characteristic appears in the phenotype if two copies are present.
-
Codominant - alleles that are both expressed in the phenotype.
/
Locus - the fixed position of a gene on a chromosome.
I
Homozygote - an organism that carries two copies of the same allele
Heterozygote - organism that carries two different alleles.
- Carrier - a person carrying an allele which is not expressed in the phenotype but can be passed onto offspring.
Monohybrid Cross: Punnett square:
Monohybrid inheritance is the inheritance of a characteristic
controlled by n single gene Parents
Parents' NN
genotypes NN un
genotypes or
Gamefes' alleles NNnn
Gamefes' alleles NNnn
N N
Fi
gen
n n Eugen
Nn Nr Nr No
Figen genotype
+
N Nu Nn NNN Nn
Gametes' alleles Nr N r
Nn Nr n Nn nn
N
NN No
Eugen genotype Nn ur
3 :
Codominance: Dihybrid cross: R- round seed
Y-yellow seed
Alleles that are codominant are both expressed in the phenotype r wrinkled seed
y-green
seed
Parents
genotypes RrYy PrYy
: Good :
1 :
blood
group O group AI blood
group
B
Gameles' allee RY
Ry riry RY
Ry riry
10 is recessive In I codominant and blood AB
give
and
group
are
Ry
.
RY rY ry
Round +
yellow :
9
RY
RRYY
RRYy RrYY
RrYy
Round 3
green-
+
wrinkled +
yellow -3
Cross Parents F1 F2 Ry RRYy RRyy RrYy Rryy
Wrinkled +
green-
I
9 3 3 :
roYY
roYy
:
RoYY
BrYy
:
rY
Monohybrid Homozygous dominant + All heterozygous 3:1
homozygous recessive Dominant : Recessive
Homozygous for one allele +
Codominant homozygous for the other 1:2:1
allele All heterozygous Homozygous for one allele:
heterozygous for the other
, Linkage
Sex-linked:
·
Genetic information for biological sex is carried on 2 sex chromosomes.
-
Females have XX chromosomes while males have XY
-
A characteristic is sex-linked when the allele that codes for it is located on a sex chromosome.
·
Y chromosome is smaller than the X chromosome and carries fewer genes. Most genes on the sex chromosomes are only carried on the
X chromosome. These are called X-linked genes.
·
Males only have one X chromosome so they often only have one allele for sex linked genes. they express the characteristic of this allele
even if it is only recessive. This makes males more likely to show recessive phenotypes than females for genes that are sex-linked.
-
Genetic disorders caused by faulty alleles on sex chromosomes include colour blindness and haemophilia. They are both carried on the X
chromosomes so they’re X-linked disorders.
Female carrier male
unuggested Female carrier + male without colour blindness = only male offspring are at
normal XWXnXNY risk of being colour blind. 3 1 OR 2 1 /
:
: :
colour blind
n =
↑ ↑ & male with
XNyn X
N
Y Female, athout
mal
XWXN XNV XNX NAnd 3: Female with colour blindness + male with colour blindness. 1 / :
Autosomal:
Autosome = chromosomes that aren’t sex chromosomes.
: Autosomal genes are genes located on the autosomes.
·
Genes on the same autosome are linked because they stay together during independent segregation of chromosomes and their alleles will
be passed on to the offspring together unless crossing over splits them up.
-
The closer together two genes are on the autosomal, the more closely they are linked because crossing over is less likely to split them up.
·
The expected ratio for a dihybrid cross is 9:3:3:1 but it is more likely to be that expected for a monohybrid cross 3:1 because the 2
autosomally-linked alleles are inherited together. This means that a higher proportion of the offspring will have their parent’s genotype and
phenotype
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