Patterns of Inheritance OCR A A-Lev

what is the genotype?

what is the genotype?

• D : genetic makeup of an organism

• describes the organism in terms of alleles it contains

what is the phenotype?

• D : visible characteristics of an organism

describe dominance v recessive

• Dominant : allele is always expressed in the phenotype, even if a different allele is present

• Recessive : alleles are only expressed in the presence of another identical allele or in the absence of a dominant allele

• Codominance : if 2 alleles of the same gene are both expressed in the phenotype of the heterozygote. genes for the blood group AB are codominant

what processes cause mutations?

• change in DNA structure

• change in number or structure of chromosomes

  • deletions

  • inversions

  • translocations

  • duplications

  • non-disjunctions

Define : Aneuploidy

• chromosome number is not exact multiple of haploid number

Define : Polyploidy

• when a diploid gamete is fertilized by a haploid gamete resulting in a triploid zygote

state 3 of each mutagen :

• physical agents

• chemical agents

• biological agents

• P : X rays, Gamma Rays, UV Light

• C : benzopyrene ( in tobacco smoke ), mustard gas, nitrous acid

• B : some viruses, transposons ( jumping genes ), food contaminants

how has sexual reroduction contributed to evolution?

• during meiosis :

• allele shuffling during crossing over in prophase 1

• independent assortment of chromosomes during metaphase/anaphase 1

• independent assortment of chromatids during metaphase/ anaphase 2

• all gametes are also genetically dissimilar. the random fusion of 2 of these gametes produces extensive genetic diversity

what are 3 examples of phenotypic variation caused by the environment

• accents

• scars

• losing a digit/limb

describe variation caused by the environment interacting with genes

• chlorotic plants (those with yellow leaves due to a lack of chlorophyll) cannot photosynthesise

• they have the gene for making chlorophyll but because the soil did not contain enough Mg, this gene is unable to be expressed

• diet is another example in animals

what does 9:3:3:1 mean?

it’s always the expected ratio for 2 unlinked genes that are on separate chromosomes

when does sex linkage occur and what are the characteristics in humans

when a gene is found on one of the sex chromosomes

characteristics include haemophilia A and colour blindness

why does sex linkage mainly effect males

if a male inherits an X chromosome from their mother with an abnormal allele they don’t have a back up on a second X chromosome that girls would have

define codominance

when both alleles present in the genotype of a heterozygous individual contribute to the individuals phenotype

what does autosomal linkage mean

• D : gene loci present on the same autosome (non sex chromosome) that are often inherited together

• inherited together because they don’t segregate independently at meiosis unless chiasmata have formed between them

• linkage reduces the number of phenotypes resulting from a cross

describe : Epistasis

• D : interaction of non-linked gene loci where one masks the expression of the other (in the phenotype)

• may work antagonistically causing masking or in a complementary fashion

• Epistasis reduces phenotypic variation

describe : recessive epistasis

• works antagonistically

• eg = inheritance of flower colour in Salvia

• 2 gene loci, A/a and B/b on different chromosomes are involved

• a pink variety genotype AAbb was crossed with a white variety aaBB. all the F1 generation has purple flowers AaBb

• Interbreeding the F1 generation gave F2 generations of purle, pink, and, white flowers in the ratio 9:3:4

• the homozygous aa is epistatic to both alleles of the gene B/b. Neither the allele B for purple nor b for pink can be expressed if there is no dominant A present.

describe dominant epistasis

• works antagonistically

• eg = inheritance of feather colour in chickes

• interaction between 2 gene loci I/i and C/c

• C/c codes for coloured feathers but the I/i gene prevents the formation of colour even when there is a C present

• individuals with at least 1 I allele have white feathers regardless

• individuals who are cc homozygous are also white

what are the ratios for epistasis and what are they a variation of?

• variation of normal 9:3:3:1 of dihybrid inheritance

• Recessive = 9:3:4

• Dominant = 12:3:1 or 13:3

• Complementary = 9:7

what does complementary epistasis require?

• enzymes that work in succession

name the type of epistasis when the presence of a dominant allele results in the same coloured characteristic regardless of the other alleles at the second locus

dominant

name the type of epistasis when the 2 second locus allele can only be expressed if there is a dominant allele on the first locus

recessive

practise Chi² test

describe : Discontinuous Variation

• D : genetic variation producing discrete phenotypes - two or more non-overlapping categories

• eg - sex, blood group, tall or short pea plants etc

• characteristics are usually monogenic

• different alleles at a single gene locus have large effects in the phenotype

describe : Continuous Variation

• D : variation that produces phenotypic variation where the quantitative traits vary by very small amounts between one group and the next

• traits showing this variation are controlled by many genes and are thus polygenic

• each gene provides an additive component to the phenotype

• different alleles at each gene locus have a small effect on the phenotype. the greater the number of gene loci contributing to the characteristic, the greater the range

• eg - foot size, cob length, leaf length, height, mass

describe how genotype and the environment can contribute to the phenotype

• although a plant with genotype AABBCC has the potential to produce the longest ears of grain, this may not happen if the plant is short of light or nutrients etc

• the expression of polygenic traits is influenced more by the environment than the expression of monogenic traits is

• what is natural selection

• what can it do

• and what are the 3 main types

• changing of allele frequency within populations over time

• NS can maintain constancy of a species as well as leading to new species

• types :

  • stabilising

  • directional

  • disruptive

describe Stabilising selection

• D : natural selection leading to a constancy within a population. intermediate phenotypes are favoured and extreme phenotypes are selected against. Alleles for extreme phenotypes may be removed from the population.

• reduces genetic variation within the population

describe : directional selection

• D : a type of natural selection that occurs when an environmental change favours a new phenotype and so results in a change in the population mean

• curve shifts

describe : disruptive selection

• favours both the extreme phenotypes and selects against the intermediate phenotype

describe : Genetic Drift

• Affects small populations

• can lead to the random loss of alleles due to chance from a population

• catastrophes such as earthquakes, floods, disease, food shortages

• can arise after a genetic bottleneck or as a result of the founder affect

describe : Genetic Botteleneck

• D : when the population size shrinks because of a catastrophe and then increases again

• reduces genetic diversity

• loss of some advantageous alleles or too many harmful alleles putting populations chance of long term survival at risk

describe : The Founder Effect

• D : when a small sample of an original population establishes in a new area

• population is likely to exhibit loss of genetic variation

• what is a population

• what is a gene pool

• what does population genetics attempt to study

• what factors can alter the amount of genetic variation within a population

• P - a group of individuals of the same species that can interbreed

• GP - the set of genetic information carried by a population (genome for individuals)

• changes in in allele frequencies over time

• population size, migration, natural selection, genetic drift anf mutation rate

what assumptions are made in the Hardy-Weinberg principle

• population is very large

• mating in the population is random

• there is no selective advantage for any genotype

• there is no mutation, migration, or, genetic drift

what is the formula for H-W Equations and what do the letters represent

• p² + 2pq + q² = 1

• p = frequency of the dominant allele

• q = frequency of the recessive allele

Describe : Isolating Mechanisms

• over time species may evolve into 2 new species

• for this to occur, the population must be split into 2 isolated population

• mutations that occur in one group cannot be interbred into the other

• in each location there will be different selection pressures and eventually different allele frequencies will results

Describe : Allopatric Speciation

• D : formation of two different species from one original species due to geographical isolation

• can be separated by lakes, river, oceans, mountains

Describe : Sympatric Speciation

• the splitting of a genetically similar population into two or more populations that undergo genetic differentiation and eventually reproductive isolation, leading to the evolution of two or more new species

• eg changes to mating behaviour or changes to genetalia

Define : Artificial Selection

• selective breeding of organisms. Involves humans choosing the desired phenotypes and interbreeding those phenotypes individually, therefore selecting the genotypes that contribute to the gene pool of the next generation

describe : Inbreeding Depression

• occurs when related individuals are crossed

• chances of inheriting 2 copies of a recessive harmful allele are increased

• genetic diversity in the gene pool reduces

Describe : Hybrid Vigour

when breeders outcross individuals belonging to 2 different varieties to obtain heterozygous individuals at many gene loci

No of commercially grown crops has greatly reduced and those left are very similar genetically. if a pathogen was introduces, all would succumb

breeders will need to rely on outcrossing more and more

Example:

A farmer breeds two different types of tomato plants together, resulting in offspring that exhibit hybrid vigor. The new plants grow faster, produce larger and tastier fruits, and are more resistant to diseases compared to their parent plants. This increased vigor is due to the combination of favorable traits from both parent plants, resulting in a stronger and more robust hybrid offspring.

what are some ethical considerations of artificial selection?

• domesticated animals are less able to defend themselves and can be easy prey

• livestock which have more lean meat and less fat are more susceptible to the cold so need housing in the winter

• some coat colours don’t camouflage