topic 8 origins of genetic variation

Question 1

explain the two main categories of variation

Answer 1

interspecific variation - members of different species intraspecific variation - members of the same species

Question 2

what are the two factors that affect variation

Answer 2

1) genetic (nature) 2) environment (nurture) most traits are a mix of the two

Question 3

how does genetics cause variation

Answer 3

result of the differences between the genomes of every individual there are 3 causes of genetic variation

Question 4

how does genetic mutation cause variation

Answer 4

changes to the sequence of base pairs in the DNA. Depending upon where they occur, they may be passed onto offspring.

although body has its own DNA repair system some mutations remain

1) point or gene mutations, miscopying of just one or few nucleotides, inc. substitutions, deletions and insertions
2) chromosomal, changes in the positions of genes within the chromosomes, more likely to make changes than point, occur when an entire chromosome is either lost during meisis or duplicated in one cell

Question 5

how does meiosis cause variation

Answer 5

the almost random mixing of genes which occurs during the formation of gametes, ensures no two egg or sperm are the same

independent assortment, chromosome pairs are distributed randomly resulting in new combinations of alleles

crossing over - process during meiosis when large multi enzyme complexes ‘cut and join’ bits of the maternal and paternal chromatids together , many new combinations of alleles arise and also potential source of mutation

Question 6

how does fertilisation cause variation

Answer 6

which gametes fuse determines the genetic makeup of the off spring

Question 7

how does environment cause variation

Answer 7

the factors the individual comes into contact with from conception to death these include: nutrition, light levels, soil pH and temperature

Question 8

what are the two types of variation

Answer 8

continuous and discontinuous both genetic and environmental factors can cause either types although textbooks say genetic variation is discontinuous and environmental are continuous

Question 9

explain discontinuous variation

Answer 9

discrete categories without any intermediate values eg. blood type, ability to roll tongue and sickle cell anaemia - these types are caused by a single or very few genes - use bar chart to represent as discrete data

Question 10

explain continuous variation

Answer 10

forms a continuous range of values eg. height, weight and skin colour they can be any value if measured precisely enough - they are either determined by environment or they are determined by a number of genes working together (polygenes)

Question 11

explain how normal distribution and continuous variation are linked

Answer 11

• continuous variation in nature takes the shape of a normal distribution (bell curve), always has the same shape but differs in height and width. two measures of the shape: the mean and standard deviation - the mean is the value at the max point ( no indication of how spread out the results are) - The standard deviation is the measure of the width of the curve, 68% results fall within 1 standard deviation of the mean. 95% within 1.96 SD of the mean. - the larger the SD the more spread out the results are

Question 12

explain pheno and genotype

Answer 12

phenotype is the physical and chemical characteristics that make up the appearance of an organism are known as its phenotype

genotype is the genetic constitution of an individual organism.

Question 13

define allele with homo and heterozygous

Answer 13

each of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

• if both alleles coding for a particular characteristic are the same then the individual is homozygous for that characteristic
• if they are different then they are heterozygous

Question 14

explain dominance and recessive phenotypes

Answer 14

dominant phenotype is caused by a dominant allele, only one allele for the phenotype is needed even in the presence of an allele for the recessive phenotype

Recessive phenotypes are only expressed when there are two alleles coding for the same recessive feature (homozygous rcessive)

in genetic diagrams represented by capital letters for the alleles representing the dominant phenotype and lowercase for the recessive

Question 15

what is a gene locus

Answer 15

specific site of the gene for a specific feature

Question 16

explain polygenetic traits

Answer 16

most traits are determined by not a single gene but an interaction of several genes (ppolygenetic) eg. eye colour is determined by a number of genes at different loci ,others have interaction with enviornment , so when looking at genetic crosses remember how simple the model is

Question 17

explain monogenic (monohybrid) crosses

Answer 17

when one gene is considered at a time in a genetic cross

use punnett square

shows you the potential alleles inherited from both parents and potential offspring

can be useful for plant and animal breeders, to find out if the feature is dominant the species must be crossed with homozygous recessive

Question 18

explain the sampling errors with genetic crosses

Answer 18

the theroetical ratios are never precise for a number of reasons

1) chance plays a large roll
2) some offspring die before they can be sampled
3) inefficient sampling techniques

fast growing plants fungi and bacteria are good with sampling techniques as they produce a large amount of offspring in a small time

Question 19

explain the work of Mendel

Answer 19

1822 - 1884

investigated the colour, shape and height of pea plants

law of segregation : first law

result of his work with monohybrid crosses, states one unit or allele for each trait is inherited from each parent, segregation takes place when gametes are formed

law of independent assortment : second law

states different traits are inherited independently of eachother , now recognise exceptions eg. gene linkage and polygenic inheritence

Question 20

explain the idea of multiple allele inheritence

Answer 20

some features have multiple alleles, means more than two possible variants , no matter how many varients any diploid individual will only inherit 2

one clear example is the ABO blood groups

Question 21

explain codominance inheritence

Answer 21

using example of ABO blood groups

3 possible alleles - A B or O

the different alleles code for the presence or no presence of the antigens on the surface of the erythrocytes (red blood cells) I^o = no angtigen , I^A = antigen A and I^B = antigen B

I^o is recessive and both I^A and I^B are dominant to it

I^A and I^B are co dominant , means both alleles are expressed and produce their proteins without mixing, if I^AI^B you have antigens A and B in surface of erythrocyte and will be blood type AB

act the same way if presented individually, no blending in phenotype which is a key feature in co dominance

Question 22

explain and describe digenic or dihybrid crosses

Answer 22

breeding experiments involving the inheritence of two pairs of contrasting characteristics at the same time

For example, say we are crossing two pea plants. The two traits we are looking at are the seed color and shape. The first seed is green and wrinkly, and the second is yellow and round.

must work out possible gametes from parents

Question 23

describe gene linkage

Answer 23

Genetic linkage describes the way in which two genes that are located close to each other on a chromosome are often inherited together

Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair color and eye color are linked, so certain hair and eye colors tend to be inherited together, such as blonde hair with blue eyes and brown hair with brown eyes.

Question 24

explain and describe evidence for gene linkage and what gene linkage means for inheritence

Answer 24

if when doing a dihybrid cross and it looks like they are inherited as one unit, shows may be gene linkage

when genes are closely linked, such as located close to eachother on the chromosome, recombination events during meiosis rarely occur

if genes are loosely linked (further apart on the chromosome) then recombination is more likely

tightness of the linkage is related to how close they are on the chromosome

The clue linkage is involved with dihybrid cross is when the expected 9:3:3:1 ratio for the hetrozygote cross does not occure

X squared test would prove interaction

Question 25

what is chromosome mapping

Answer 25

using linked genes we can find out the genetic map of a chromosome, cross over value must be worked out

crossover value = number of recombinant offspring (x100) / total number of off spring

using this formula genes which are closely linked have lower cross over values

genes further apart produce larger values

these values can be used to build up chromosome maps, production has been speeded up by computers and bioinformatics

• vital for genetic engineering

Question 26

what is sex linkage

Answer 26

• Sex in humans is determined by the combination of our 23rd chromosome pair. These can be X or Y.
• Females have the chromosomes XX.
• Males have the chromosomes XY.

Some disorders are sex linked. This means that the genes for it are carried on the X or Y chromosomes.

As the Y is much shorter than X, for much of its length there is no homologous gene to X.

This means that recessive alleles on the X chromosome have no dominant allele to suppress it and so are more likely to appear in males.

• These include:
• Haemophilia
• Colour Blindness
• Duchenne Muscular Dystrophy
• Menkes disease (kinky hair disease)

Question 27

explain dominance hierarchy with multiple alleles and co dominance

Answer 27

• In some cases where there are more than two alleles each may be dominant to some and recessive to others. This is a dominance hierarchy.
• The example given in the textbook is coat colour in rabbits which is (most dominant first):

Agouti (CA) > Chinchilla (CCh) > Himalayan (CH) > Albino(Ca)

Question 28

explain the Hardy-Weinberg Principle

Answer 28

• This allows us to calculate the frequencies of alleles of a particular gene in a population.
• This assumes that the proportion of dominant and recessive alleles remains constant as long as the following conditions are met:
• No mutations arise
• There is no flow of alleles into or out of the population
• There is no selection acting upon them
• The population is large
• Mating is random within the population

Question 29

Tay-Sachs disease is a genetic disorder caused by a recessive allele. The incidence is around 1 in 3,000 individuals suffer from the disorder.

How many people are carriers for the allele?

T = healthy allele

T = Tay-Sachs allele

Answer 29

Question 30

Marfan Syndrome is a genetic disorder caused by a dominant allele. The incidence is 1 in 4,000.

How many people are homozygous dominant?

Answer 30

Question 31

explain what genetic diversity is

Answer 31

• Each of these species is different to every other.
• Even within species there are huge differences both that we can see and that we cannot.
• This diversity is all down to our genes.
• Diversity arises from differences in the genetic code.
• There may be more than one version of a gene. These versions are known as alleles.
• Greater diversity is a good thing for a population as it means they have a greater ability to adapt to a change in their environment. This is because it is more likely that some individual will have the right characteristic to survive.

Question 32

explain how genetic diversity arises

Answer 32

• There are a number of ways to change the diversity of a species. Some are natural and some man-made.
• Selective Breeding
• The Founder Effect
• Genetic Bottlenecks

Attempts to change the genetic diversity of the human species are known as eugenics

Question 33

explain selective breeding

Answer 33

• Also known as artificial selection.
• This involves identifying individuals with preferable characteristics, and therefore preferable alleles, and using them to parent the next generation. Individuals with non-preferable characteristics are not allowed to reproduce.
• Over several generations only the preferable alleles remain in the population.

This method is used to improve the yield or quality of domesticated plants and animals or to increase useful qualities such as disease resistance

Question 34

explain the founder effect

Answer 34

• This is where a few individuals from a population migrate to a new, separate region and form a new population.
• As with any sample these may not be representative of the population as a whole.
• As the population grows it will have less genetic diversity than the parent population.
• The new population may be quite different and in time develop into a new species.
• This is most often seen when islands are colonised for the first time.

Question 35

explain the genetic bottleneck effect

Answer 35

• Sometimes a species may suffer a dramatic drop in numbers either from a natural disaster or, more recently, from man’s intervention.
• The remaining few will have fewer alleles and so when the population recovers it will have a much lower genetic diversity.
• The northern elephant seal Mirounga angustirostris was hunted until, in 1900, only 20 remained in Mexico. They have recovered but now have a very low diversity.

Question 36

explain who can recieve blood group donations and why

Answer 36

the antigens on your erythrocytes have corresponding antibodies in your blood plasma

A = B

B = A

AB = none

O = AB

O is universal donor as has no antigens so causes no antigen antibody reaction, O can only recieve O

people with type AB can recieve any blood as they have no AB antibodies , universal recipients, however AB can only be given to AB

Question 37

what is an autosome

Answer 37

an autosome are chromosomes which are not concerned with sex determination