7A - genetics

Question 1

what is a chromosome?

Answer 1

a long section of DNA wound around a protein called a histone

Question 2

what is a gene?

Answer 2

a length of DNA that codes for a single polypeptide or protein

Question 3

what is a locus?

Answer 3

the fixed position of a gene on a chromosome

Question 4

what is an allele?

Answer 4

a different form of a gene

Question 5

what do different alleles of a gene have?

Answer 5

slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome

Question 6

examples of alleles:

Answer 6

-one of the genes for coat colour in horses is Agouti
-this gene for coat colour is found on the same position on the same chromosome for all horses
-hypothetically there are two different forms (alleles) of that gene found in horses: A and a
-each allele can produce a different coat colour:

allele A → black coat
allele a → chestnut coat

Question 7

how do chromosomes of eukaryotic cells occur?

Answer 7

in homologous pairs (there are two copies of each chromosome)
↳ as a result cells have two copies of every gene

Question 8

what does having two copies of each gene mean?

Answer 8

there can be different allele combinations within an individual

Question 9

what is the genotype?

Answer 9

all of an organisms genetic material / the alleles of a gene possessed by that individual

Question 10

homozygous

Answer 10

when the two allele copies are identical in an individual / you’ve inherited the same alleles of a gene from each of your parents

Question 11

heterozygous

Answer 11

two different versions of the same gene

Question 12

what is the phenotype?

Answer 12

the physical expression of the genotype that is affected by environment and genotype

Question 13

what is a dominant allele?

Answer 13

alleles that are always expressed in the phenotype

Question 14

recessive alleles

Answer 14

only expressed in the phenotype if no dominant allele is present

Question 15

codominance

Answer 15

when both alleles can be expressed in the phenotype at the same time

Question 16

how to write the codominance genotype:

Answer 16

when writing the genotype for codominance the gene is symbolised as the capital letter and the alleles are represented by different superscript letters, for example IA

Question 17

example of codominance:

Answer 17

the gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O

allele A results in blood type A (IAIA or IAIO) and allele B results in blood type B (IBIB or IBIO)

if both allele A and allele B are present in a heterozygous individual they will have blood type AB (IAIB)

blood type O (IOIO) is recessive to both group A and group B alleles

Question 18

what happens when a homozygous dominant individual is crossed with a homozygous recessive individual?

Answer 18

the offspring are called the F1 generation → all of the F1 generation are heterozygous

Question 19

what happens if two individuals from the F1 generation are then crossed?

Answer 19

the offspring they produce are called the F2 generation

Question 20

what are the two types of linkage in genetics?

Answer 20

sex linkage and autosomal linkage

Question 21

how many sex chromosomes are there?

Answer 21

X and Y

Question 22

male chromosomes and female chromosomes:

Answer 22

women: XX
men: XY

Question 23

what is sex linkage?

Answer 23

-some genes are found on a region of a sex chromosome that is not present on the other sex chromosome
-as the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes

Question 24

where are most sex linked genes found?

Answer 24

on the longer X chromosome

Question 25

example of sex linked diseases:

Answer 25

haemophilia

Question 26

how are sex-linked genes represented?

Answer 26

-by writing the alleles as superscript next to the sex chromosome -for example a particular gene that is found only on the X chromosome has two alleles G and g -the genotype of a heterozygous female would be written as XGXg -a males genotype would be written as XGY

Question 27

what is autosomal linkage?

Answer 27

-this occurs on the autosomes (any chromosome that isn’t a sex chromosome) -linkage between genes affects how parental alleles are passed onto offspring through the gametes -two or more genes on the same chromosome do not assort independently during meiosis -these genes are linked and they stay together in the original parental combination

Question 28

what does mon hybrid inheritance look at?

Answer 28

it looks at how the alleles for a single gene are passed on from one generation to the next (genetic crosses of single gene combinations)

Question 29

what happens when two individuals sexually reproduce?

Answer 29

there is an equal chance of the zygote inheriting either allele from their parent

Question 30

why are genetic diagrams often used?

Answer 30

present the information of a homologous pair’s reproductionin a clear and precise manner so that predictions can be made

Question 31

examples of a genetic cross diagram for monohybrids:

Answer 31

a punnet square

Question 32

genotypes that genetic diagrams produce:

Answer 32

-the predicted genotypes that genetic diagrams produce are all based on chance -there is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions

Question 33

three genotypes for females (sex linked)

Answer 33

XAXA = unaffected XAXa = carrier XaXa = affected

Question 34

two phenotypes for males (sex-linked)

Answer 34

XAY = unaffected XaY = affected

Question 35

can men carry x linked traits?

Answer 35

no, males only pass y chromosomes on to their sons

Question 36

EXAMINER TIP:

Answer 36

Make sure to include all of your working out when constructing genetic diagrams. It is not enough just to complete a punnett square, you need to show that you have thought about the possible gametes that can be produced by each parent.Also, remember to state the phenotype as well as the genotype of the offspring that result from the cross. Read the questions carefully when answering sex-linked inheritance questions – is the question asking for a probability for all children or is it asking about a specific gender (boys or girls).

Question 37

what do dihybrid crosses look at?

Answer 37

how the alleles of two genes transfer across generations

Question 38

how to write the genotypes for a dihybrid genetic cross:

Answer 38

write the two alleles for one gene, followed immediately by the two alleles for the other gene. Do not mix up the alleles from the different genes if there was a gene with alleles Y and y and another gene with alleles G and g an example genotype for an individual would be YyGg

Question 39

what is epistasis?

Answer 39

when one gene can affect the expression of another gene (two genes on different chromosomes affect the same feature

Question 40

TIP:

Answer 40

genetics questions you may notice that crosses involving autosomal linkage predict offspring that all have the same combination of characteristics as their parents. In reality recombinant offspring are often produced; this is due to crossing over during meiosis. Crossing over breaks the linkage between the genes and recombines the characteristics of the parents. Exam questions often ask students to explain the appearance of recombinant offspring in crosses that involve linkage; crossing over is the most likely explanation

Question 41

recombinant

Answer 41

offspring that have a different combination of characteristics to their parents

Question 42

what is a test cross used for?

Answer 42

to deduce the genotype of an unknown individual that is expressing a dominant phenotype

Question 43

results of a monohybrid test cross:

Answer 43

If no offspring exhibit the recessive phenotype then the unknown genotype is homozygous dominant If at least one of the offspring exhibit the recessive phenotype then the unknown genotype is heterozygous

Question 44

results of a dihybrid test cross:

Answer 44

If no offspring exhibit the recessive phenotype for either gene then the unknown genotype is homozygous dominant for both genes If at least one of the offspring exhibit the recessive phenotype for one gene but not the other, then the unknown genotype is heterozygous for one gene and homozygous dominant for the other If at least one of the offspring exhibit the recessive phenotype for both genes then the unknown genotype is heterozygous for both genes

Question 45

Examiner Tips and Tricks

Answer 45

Make sure before you start a test cross you think about the following: how many genes are there, how many alleles of each gene are there, which is the dominant allele, what type of dominance is it and is there linkage or epistasis between genes?