Inheritance Flashcards
What is a gene
Information packed in discrete particles which can be passed on from parents to offsprings
Offsping contain 2 copies of each gene - one from mother one from father
What is phenotype
The observable characteristics of an individual
What does a phenotype result from
Genotype and the environment
What is genotype
Combination of alleles of the relevant gene
What is atavism
The reappearance of a trait in a pedigree/family
Atavism is bet described by
the dominant and recessive nature of alleys in determining phenotype
State Mendel’s first law
Law of segregation
Genes segregate at meiosis so that each gamete only contain one of the 2 genes possessed by the parent
State Mendel’s second law
Law of independent assortment
Alleles of different genes on nonhomologous chromosomes assort independently during gamete formation. Individual chromosomes have no effect on where the other chromosome go during meiosis
What is a test cross
Cross with homozygous recessive to determine unknown genotype of parent
If a homozygous recessive is test crossed with a homozygous dominant, what will the offspring phenotype ration be
All of one phenotype
If a homozygous recessive is test crossed with a heterozygous, what will the offspring phenotype ratio be
1:1
What are causes of deviation from simple Mendelian ratios (4)
Lethal allele
Polymorphic
Incomplete dominance
Complete dominance
What is it meant by polymorphic
A gene coded for many alleles, leading to various phenotypes
What is incomplete dominance
When one allele/phenotype does not completely dominate over the other, resulting in the offspring showing a phenotype that is the mix of the parents (heterozygous genotype)
What is co-dominance
1 gene with 2 alleles expressed side by side and the organism shows the phenotypic effects of both alleles equally
Give an example of co-dominance
ABO blood antigen system
- the ABO blood groups in humans are determined by 3 alleles of a single gene
- A person’s blood group may be one of 4 types: A, B, AB, O and these letters refer to 2 carbohydrates A and B
- A person’s blood cell may have
carb A = blood type A
carb B = blood type B
neither carb = type O
carb A and B = type AB which shows co-dominance
What is epistasis
Where one gene at one locus alters the phenotype expression of a gene at a second locus
Outline the basic metabolic pathway of epistasis
Gene A determines whether or not color is produced - (A) or nor (a)
Gene B codes for coat color - (B) or (b)
Gene B is said to be under epistatic control of gene A
What is a polygenic trait
A phenotype controlled by many genes that have an additive effect
Examples of polygenic traits (3)
skin color, weight, height
How is a polygenic trait typically distributed in a population
Normal distribution
The more genes involved, the higher number of phenotypic classes
Incomplete dominance, complete dominance, epistasis and polygenic trait are examples of what?
How some phenotypic traits are affected by several loci
Discuss how environment factors may affect phenotype and provide examples
It can smooth out differences among phenotypes
eg. diseases, height/weight, nutrition, life expectancy
Hydrangeas are blue in acidic soil but pink in alkaline soil
Explain how the environment smooths out the phenotype of wheat height
Height of weight is discrete with 3 values but the environment will cause wheat to grow in between these 3 values and can cause in normal distribution
What is pleiotropy
When one gene has many effects
Examples of pleiotropy (3)
- Sickle cell produces many symptoms
- colouration of pattern and cross eyes of siamese cats produced by the same gene
- eye colour, wing length and body hair traits in Drosophila all affected by one gene
Homogametic
having 2 of the same sex chromosomes
In humans = female
Heterogametic
having 2 different sex chromosomes
In humans = male
Define linked genes
When 2 genes are close together on the same chromosome and do not assort independently
What is a sex-linked gene
A gene located on either sex chromosomes
What is a hemizygote
A diploid organism or cell with only 1 copy of the chromosome
In a X-linked gene inheritance, how will the mutant allele be inherited if the father carried the mutant allele mated with a dominant homozygote mother
The daughters will have the normal phenotype but will be carriers for the mutation
No mutant allele will be passed onto sons
If a carrier female for an X-linked disorder mates with a normal male, how will the mutation/disorder be passed on
Half the sons and half the daughters will receive the mutation. 50% chance that daughters will be carriers like their mother but the 50 % chance that sons will have the disorder
If a carrier female for an X-linked disorder mates with a male who also has the disorder, how will the disorder be passed on
Half the sons and half the daughters will have the disorder regardless of sex
Daughters free of disorder will be carriers
Sons free of disorder will be free of the mutated allele
Examples of X-linked inheritance
Eye color in Drosophila
Red-green color blindness
Examples of X-linked disorders
Duchenne muscular dystrophy
Hemophilia
Linked genes
Genes located near each other on the same chromosome which tend to be inherited together in genetic crosses
If the occurrence of parental types has a frequency greater than 50%, what does that indicate
linked genes
Explain how crossing over lead to recombinants
During late prophase I of meiosis, the 2 chromatids of a tetrad (1 maternal, 1 paternal) cross over at random points and swap genetic material
What is the recombinant frequency
The proportion of recombinants
What determines the recombinant frequency
The distance between 2 genes. The further the 2 genes are, the higher the probability that a chiasma will form between then, the 2 genes will cross over therefore higher recombinant frequency
What type of linkage results in parental types only and no recombinants
complete linkage
What type of linkage results in recombinants
Incomplete linkage
What does 1 map unit equal to
1% recombinant frequency
Recombinant frequency of distant/unlinked genes
50%
What does a recombinant frequency of 0%-50% equal to
Close genes
How is the recombinant frequency calculated
of recombinant/total number x 100
Define gene pool
localized group of individuals of the same species
‘The total aggregate of genes (and their alleles) in the population at one time’ refers to what
Gene pool
5 assumptions of Hardy-Weinberg Equilibrium
Large population size Random mating No mutations No migration No natural selection
Why does genetic drift occur more rapidly in small populations
The smaller the population, the more likely that chance events can cause allele frequency fluctuations from one generation to the next
‘A random change in allele frequencies due to sampling error over generations’ describes what
Random genetic drift
Define bottleneck effect
Sharp reduction in population size due to environmental events or human activities
What is founder effect
Reduced genetic variation from original population when a new colony/population is established by a small number of individuals from the original population
Natural selection (2)
Process whereby organisms better adapted to their environment tend to survive and produce more offspring
Alters gene frequency
What is the only mechanism that consistently causes adaptive evolution
Natural selection
State the 3 types of natural selection
Stablising
Directional
Disruptive
Stabilising selection (2)
Favours average/intermediate phenotype and act against extreme phenotypes
Results in reduction in variation but does not change the mean
Example of stabilizing selection
head size at childbirth
Directional selection
Favors one extreme phenotype
Results in peak shifting in one direction and changes the mean value towards one extreme
Example of directional selection
Long necks of giraffe
Disruptive selection
Favors the 2 extremes
Result in 2 peaks
Example of disruptive selection
2 bill morphologies displayed by West African blacked bellied seek crackers
What is sexual selection
A mode of natural selection where members of one biological sex choose mates of the other sex to mate
Tends to be females choosing mates with favourable traits
What is frequency dependent selection
Evolutionary process where the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population. In positive frequency-dependent selection, the fitness of a phenotype increases as it becomes more common.
Which mode of selection favors the extremes
frequency dependent selection
Artificial selection (Not a type of natural selection)
Human selection of preferred phenotype of a species resulting in altered gene frequency
Gene flow/migration affects on the population (4)
Brings more alleles
Changes proportions of existing alleles
Changes population size
Makes 2 populations more similar
Cline
Environmental gradient across extended geographical range resulting in gradual change in proportions as different environments exert different selection pressures
