Module 6.2 Flashcards
Patterns of inheritance
what is the genotype
the genetic makeup of an organism
what is the phenotype
the visible characteristics of an organism
causes of phenotypic variation
influenced by both genotype and environment
how do genetic factors influence phenotypic variation
mutations cause changes to the genetic material
sexual reproduction may also lead to genetic variation
How does sexual reproduction lead to genetic variation
meiosis produces genetically different gametes
genetic variation may result from:
allele shuffling during crossing over in prophase 1
independent assortment of chromosomes during metaphase 1
independent assortment during metaphase 2
random fusion of gametes at fertilisation creates diversity as any male gamete can combine with any female gamete
example of phenotypic variation cause solely by the environment
loosing a digit or limb due to injury
speaking with a particular regional dialect
examples of phenotypic variation caused by environment interacting with genes
chlorosis
height
what is chlorosis
plants kept in dim light or in soil with insufficient magnesium do not develop enough chlorophyll so cannot photosynthesise
have the genotype for chlorophyll production but environment prevents the expression of the genes
How is height influenced by both genes and environment
an individual may have the genes to be tall however if they do not receive sufficient nutrition they will not be able to reach this height
What is monogenic inheritance
when a characteristic is governed by one gene that has two distinct alleles
what is P1
the parental generation
one parent is homozygous for the dominant trait and one parent is homozygous for the recessive trait
what is F1
the first filial generation
all offspring are heterozygous and phenotypically identical
what is F2
second filial generation
4 different genotypes
3 display dominant phenotype
1 displays recessive phenotype
phenotypic ratio for monogenic inheritance
3:1
3 dominant allele displayed in the phenotype
1 recessive allele shown in the phenotype
how can we ascertain the genotypes of phenotypically similar individuals
test cross
what is dihybrid inheritance
The simultaneous inheritance of two characteristics
the alleles of the two genes are inherited independently of each other so each gamete had one allele for each gene locus
What is the phenotypic ratio for dihybrid inheritance
9:3:3:1
9 both dominant alleles are displayed in the phenotype
3 one dominant allele and one recessive allele is displayed in the phenotype
1 both recessive alleles are displayed in the phenotype
What is multiple alleles
When a characteristic has three or more alleles at a specific gene locus
any individual can only posses two alleles - one on each gene locus
example of multiple alleles
ABO blood groups
what is codominance
when both alleles present in the genotype of a heterozygous individual contribute to the individuals phenotype
example of codominance
the four blood groups A, B, AB and O
A and B are both dominant to O
A and B are codominant so if both A and B are present in the genotype they with both contribute to the phenotype - AB blood group
what is sex-linkage
when the gene is present on one of the sex chromosomes
why does sex-linkage occur
autosomal chromosomes are fully homologous (match for length and contain the same genes at the same loci)
X and Y chromosomes are not fully homologous so there are areas on the X chromosome which have no partner allele on the Y chromosome
what is autosomal linkage
where gene loci on the same autosome are often inherited together
how is autosomal linkage inherited without crossing over
genes are inherited as one unit
what is the phenotypic ration of autosomal linkage when no crossing over occurs
3:1
3 display both dominant alleles in the phenotype
1 displays both recessive alleles in the phenotype
what occurs from crossing over
recombinant gametes
what increases the chance of recombinant gametes
the further apart the two gene loci on a chromosome are
What is epistasis
interaction of non-linked gene loci where one masks the expression of another
what is recessive epistasis
the homozygous presence of a recessive allele at the first locus prevent the expression of another allele at a second locus
the alleles at the first locus are epistatic to those at the second locus which a hypostatic to those at the first locus
What is dominant epistasis
the homozygous or heterozygous presence of a dominant allele at the first locus prevents the expression of another allele at a second locus
phenotypic ratio of recessive epistasis
9:3:4
9 dominant allele of second gene loci is displayed in the phenotype
3 recessive allele of second gene loci is displayed in the phenotype
4 recessive allele of the first gene loci is displayed in the phenotype
phenotypic ratio of dominant epistasis
12:3:1
13 dominant allele of first gene loci is displayed in the phenotype
3 dominant allele of second gene loci is displayed in the phenotype
1 recessive allele of both gene loci is displayed in the phenotype
when can the Chi-squared test be used
when data is in categories
strong biological theory to use to predict expected values
sample size is large
data is raw counts
no zero scores
how to calculate degrees of freedom in the chi-squared test
number of catrgories-1
what is O in chi squared
observed value
What is E in chi squared
expected value
what is discontinuous variation
When there are distinct and discernible classes with no intermediates between groups
how many genes influence discontinuous variation
usually determined by the alleles of a single gene locus
monogenic
what is continuous variation
when the data has a range of values with a full range of intermediates in-between the minimum and maximum value
how many genes influence continuous variation
many genes involved - polygenic
alleles of each gene may contribute a small amount to the phenotype - additive effect on the phenotype
what increases the range of continuous variation
the greater number of gene loci contributing to the determination of the characteristics
what factors can effect the evolution of a species
stabilising selection
directional selection
genetic drift
genetic bottleneck
founder effect
what is stabilising selection
natural selection leading to constancy within a population
intermediate phenotypes are favoured and extreme phenotypes selected against
alleles for extreme phenotypes may be removed from the population
when does stabilising selection occur
when organisms environment remains unchanged and it favours intermediate phenotypes
what does stabilising selection do
reduces genetic variation as alleles for extreme phenotypes may be removed from the population
what is directional selection
a type of natural selection that occurs when an environment change favours a new phenotype and so results in a change in the population mean
when does directional selection occur
when environment changes so there is a selectional advantage given to phenotypes different from the original mean phenotype
What is genetic drift
when the relative frequency of genotypes in a small population varies due to the disappearance of genes as individuals die or don’t reproduce
how does genetic drift occur
if a population descends from a small number of parents the gene pool lacks variation
mutations provide neither advantage or disadvantage so selection pressure does not act upon them
chance events such as earthquakes, flood or disease lead to the death of many in the already small population so alleles may disappear
when the population recovers it will have less genetic diversity
what can genetic drift be the result of
genetic bottleneck
founder effect
what is a genetic bottleneck
a sharp reduction in the size of a population due to environmental catastrophes, disease of human activities
reduces genetic diversity so as the population expands it is less genetically diverse than before
what is the founder effect
when a small sample of an original populations establishes in a new area
its gene pool is not as diverse as that of the parent population
what is the Hardy-Weinberg principle used for
determining changes in allele frequencies within a population over time
what does the Hardy-Weinberg principle assume
the population is large enough to make sampling error negligible
mating occurs at random
there is not selective advantage for any genotype so no selection
there is no mutation, migration or genetic drift
what is p in the Hardy-Weinberg principle
frequency of the dominant allele
what is q in the Hardy-Weinberg principle
frequency of the recessive allele
what is speciation
the splitting of a genetically similar population into two or more populations that undergo enough genetic differentiation to lead to reproductive isolation so two or more new species are created
what are the main types of isolating mechanism
geographical isolation
reproductive isolation
what is geographical isolation
if populations are separated and isolated from each other by geographical features which act as barriers to the gene flow between the populations
what does geographical isolation result in
allopatric speciation
how does allopatric speciation occur
the isolated populations are subject to different selection pressures in the two different environments
undergo independent changes to allele frequencies and or chromosome arrangements in their gene pool so new species arise
what is reproductive isolation
when biological and behavioural changes in a species may lead to isolation of one population from another
e.g. change in chromosome number
change in time of activity
change in courtship behaviour
change in genitalia
what does reproductive isolation lead to
sympatric speciation
what is sympatric speciation
when reproductive isolation leads to the formation of new species
