Inheritance

Question 1

define gene

Answer 1

sequence of DNA bases that codes for a specific polypeptide

Question 2

where are the genes for particular traits found?

Answer 2

found in the same location on the chromosomes (locus) in different individuals of the same species

Question 3

what are the three main characteristics of a gene?

Answer 3

they can separate and combine

they can mutate

they code for the production of specific polypeptides

Question 4

what does a diploid individual contain?

Answer 4

homologous pairs of chromosomes, one chromosome copy from mother and one from father

therefore have two copies of each gene

Question 5

define alleles

Answer 5

they are alternative forms of genes occupying a similar locus on homologous chromosomes

Question 6

what are the three different allele combinations for any one gene?

Answer 6

heterozygous - having different alleles for a given gene (i.e. a dominant and a recessive allele are present together)

homozygous dominant - having two dominant alleles for a given gene

homozygous recessive - having two recessive alleles for a given gene

Question 7

what is the gene pool?

Answer 7

all the alleles in a population at any one time are called the gene pool

Question 8

define genotype

Answer 8

the genetic make-up of an individual; all of the alleles that they possess

Question 9

define phenotype

Answer 9

the observable characteristics of an organism determined by the genotype

in the simplest situations, a particular characteristic is controlled by a single gene with two alleles

therefore, if an organism is homozygous dominant or heterozygous for a particular characteristic; the dominant allele will always be expressed in the phenotype

the phenotype for a homozygous recessive individual would be different

Question 10

define monohybrid inheritance

Answer 10

it is the inheritance of a single gene, such as that controlling plant height or seed colour

Question 11

describe Gregor Mendel’s investigation on pea plants

Answer 11

Mendel chose pairs of contrasting characteristics such as tall or dwarf plants, round or wrinkled seeds and yellow and green seeds

he was fortunate that his choice of characteristics were controlled by single genes and were clear cut and easy to tell apart

Question 12

why are pea plants a useful choice for inheritance experiments?

Answer 12

they are easy to grow

can self or cross fertilise

can produce flowers and fruits in the same year

Question 13

give the 5 instructions for genetic crosses

Answer 13

look at the parents: are they homozygous or heterozygous

choose suitable symbols for the alleles (unless already provided)
choose the first letter of the contrasting features if possible
use upper case for dominant and lower case for recessive

clearly label the parents; state their phenotypes, genotypes, and state the gametes produced by each
circle the gametes
remember there will only be one allele for a given gene in each gamete

draw a punnet square to cross the gametes

state the phenotypes and genotypes for the offspring results, with ratios

Question 14

how would Mendel ensure that offspring produced were only from the parents above (page 5 in booklet)?

Answer 14

pollen transferred by hand - receiving flower could be in a bag to prevent further cross-pollination

remove anthers before they mature so no self-pollination

Question 15

describe Mendel’s first ‘law of inheritance’

Answer 15

the characteristics of an organism are determined by (factors) alleles, which occur in pairs

only one allele of a pair is present in each gamete

Question 16

describe continuous variation

Answer 16

the characteristic often has a range of values (usually the bell-shaped normal distribution curve)

controlled by a number of genes (polygenic), e.g. if height is influenced by two or more genes, then they have the potential to grow tall

environmental factors have an effect

Question 17

describe discontinuous variation

Answer 17

the characteristic has distinct categories controlled by a single gene

environmental factors have no effect

Question 18

state whether these following characteristics are continuous or discontinuous:
tongue rolling
ear lobes attached/unattached
skin colour
height
hair colour
blood type

Answer 18

(in order)
D
D
C
C
C
D

Question 19

describe the ‘backcross’/test cross method

Answer 19

it is used in genetics to determine whether a particular dominant characteristic observed in an organism is caused by one or by two dominant alleles (heterozygous or homozygous dominant)

the unknown is always crossed with an individual displaying the recessive phenotype

Question 20

describe pure breeding

Answer 20

two organisms of the same phenotype that, when bred together, produce offspring with the same phenotype (homozygous)

(example on page 8)

Question 21

what is codominance?
(page 9 in booklet)

Answer 21

is a condition in which both alleles for a gene are expressed when present in a heterozygote
both alleles are written using a capital letter

Question 22

what is incomplete codominance?
(page 9 in booklet)

Answer 22

where heterozygous individuals produce a phenotype intermediate of the parental phenotypes

the heterozygous condition is somewhere in between

both alleles are written using a capital letter

Question 23

what is dihybrid inheritance?
(page 10 in booklet)

Answer 23

involves the inheritance of two unlinked genes (genes found on different chromosomes)

independent assortment of these genes produces recombinants (different allele combinations in the gametes)

Question 24

describe Mendel’s second law (law of independent assortment)

Answer 24

either a pair of alleles may combine randomly with either of another pair

Question 25

give two key terms of chi-squared

Answer 25

expected - based on ratios gained through genetic crosses observed - actual ratios

Question 26

describe the idea behind chi-squared

Answer 26

genetic crosses give expected offspring ratios however, when the cross is actually carried out the ratio of offspring produced could be different

Question 27

what does a chi-squared test work out?

Answer 27

if the expected ratio is significantly different from the observed ratio

Question 28

define null hypothesis

Answer 28

there is no significant difference between the observed and the expected

Question 29

describe the use of null hypothesis in chi-squared

Answer 29

if there is no significant difference between the observed and expected ratios then we accept our null hypothesis - any difference is down to chance if there is a significant difference between the observed and the expected then we reject the null hypothesis - other factors must be having an influence

Question 30

give the steps on how to use the chi-squared equation: χ² = Σ ((O - E)² / E) (look at pages 14-15 in booklet)

Answer 30

equation can be solved using a table (sometimes you may need to add columns to the table) once chi-squared value is worked out, we use the chi-squared table to work out if the value is significant or not the value for the degrees of freedom is one less than the number of categories (i.e. phenotypes) we've used aka: no. of phenotypes - 1 we always compare our value to the value at the 5% significance level (shown usually as 0.05) unless the question says otherwise if the chi-squared value is less than the value in the table, we accept the null hypothesis, therefore any difference is due to chance if the chi-squared value is greater than the value in the table, we reject the null hypothesis, therefore other factors are influencing

Question 31

what are the two types of chromosomes?

Answer 31

autosomes - any chromosome that is not a sex chromosome sex chromosomes - a chromosome concerned in determining the sex of an organism, typically one of two kinds in humans, they are similar in one sex (f) and dissimilar in the other (m)

Question 32

out of the 23 pairs of chromosomes in humans, which are autosomes and which are sex chromosomes?

Answer 32

first 22 are autosomes last pair are the sex chromosomes

Question 33

what does it mean when alleles are sex-linked?

Answer 33

some alleles are carried on the x-chromosome

Question 34

what is the effect on males in sex-linked inheritance?

Answer 34

the male Y chromosome is much smaller than the X and so carries far fewer genes for most genes carried on the X chromosome in the male there is only one allele which must be expressed therefore, in the male any recessive genes on the X chromosome will be expressed in the phenotype

Question 35

if a male with a sex-linked disease had a son, what are the chances of him having the disease?

Answer 35

no chance the father will only pass his Y chromosome to his son males can only inherit X-linked genes from their mother

Question 36

if a male with a sex-linked disease had a daughter, what are the chances of her having the disease?

Answer 36

females can inherit X-linked genes from their mothers or fathers but require 2 recessive alleles for a disorder to show in the phenotype

Question 37

why might women with a carrier father and unaffected mother want genetic counselling before having children?

Answer 37

to find out if they are carriers to know the rises to possible sons and to decide whether to screen embryos for the disorder

Question 38

describe haemophilia and a possible symptom of it (page 18 in booklet)

Answer 38

the individual cannot produce enough of one particular blood clotting protein slow persistent bleeding

Question 39

describe duchenne muscular dystrophy and a possible symptom of it (page 19 in booklet)

Answer 39

the dystrophin gene codes for the protein dystrophin, which stabilises cell membranes of muscle fibres the disease leads to the progressive weakening and wasting of the muscles

Question 40

why might there be a low number of children who have sex-linked diseases through affected parents?

Answer 40

affected parents might die before they can reproduce or decide they don't wish to conceive and pass the disease onto offspring

Question 41

describe linkage

Answer 41

a chromosome contains a linear sequence of genes which are all linked and are usually inherited together however, if genes are on different chromosomes they are not linked Mendel's ratios (3:1, 9:3:3:1) only apply when genes are on different chromosomes - not linked

Question 42

The genotype AaBb has two genes (A and B) that are found on the same chromosome - linkage what gametes can be produced by this genotype of crossing over does NOT disrupt the linkage?

Answer 42

AB ab

Question 43

The genotype AaBb has two genes (A and B) that are found on the same chromosome - linkage what gametes would be produced if crossing over did occur - incomplete linkage? what percentage of crossing over affects linkage?

Answer 43

AB, ab - many of these because they are the parental combinations aB, Ab - fewer of these because recombination is rare crossing over disrupts linkage only 5-10% of the time

Question 44

describe incomplete linkage

Answer 44

crossing over can affect the linkage of genes and leads to incomplete linkage

Question 45

how likely are genes going to be affected by crossing over in linkage?

Answer 45

if they are further apart on a chromosome

Question 46

summarise linkage and incomplete linkage (page 22 in booklet)

Answer 46

linkage: genes are inherited together crossing over is very unlikely to separate them as the genes are too close together on the same chromosome incomplete linkage: genes are on the same chromosome bat are far apart genes could be separated due to crossing over and not be inherited together (rare (5-10%))

Question 47

define mutation

Answer 47

an unpredictable change in the genetic material of an organism

Question 48

give the two types of mutations

Answer 48

gene mutations - affect single bases within a gene chromosome mutations - cause changes in the structure or number of whole chromosomes (many genes are affected)

Question 49

why are mutations important?

Answer 49

they are spontaneous random events that provide an important source of genetic variation

Question 50

describe mutation rates

Answer 50

rates are low but in organisms with short life cycles and frequent cell division they are more frequent

Question 51

when do chromosomal mutations most often occur?

Answer 51

during crossing over in prophase I and non-disjunction during anaphase I and anaphase II

Question 52

give some examples of mutagens

Answer 52

radiation: X-rays, gamma radiation, UV light chemicals: polycyclic hydrocarbons in cigarette smoke

Question 53

why is it incorrect to say that mutagens cause mutations?

Answer 53

mutagens do not cause mutations but increased exposure to mutagens increases the rate of mutations occurring

Question 54

define carcinogens

Answer 54

mutagens that increase the rate of cancer

Question 55

what genes regulate cell division in humans and what happens when they become mutated?

Answer 55

proto-oncogenes and tumour-suppressor genes if mutated they form oncogenes

Question 56

what are oncogenes involved in?

Answer 56

uncontrollable cell division and can lead to cancer

Question 57

describe benign and malignant tumours

Answer 57

benign - go slowly and do not spread malignant - made up of cancerous cells and spread

Question 58

what chemicals does tobacco smoke contain that is harmful to human health?

Answer 58

tar (toxic chemicals) nicotine (addictive) carbon monoxide

Question 59

what does tar contain and what cancer may develop?

Answer 59

carcinogens which target the DN in the cells of the alveoli if mutation occurs in proto-oncogenes/tumour-suppressor genes, lung cancer may develop

Question 60

what three things could a change in a single base of a gene result in?

Answer 60

change in DNA base sequence, resulting in a change in triplets/codons for amino acids different amino acid sequence produced in translation bonds form in different places and result in a different shaped protein (may not function)

Question 61

use the enzyme melanin to give an explanation of how a mutation in the gene coding for this enzyme results in a lack of pigmentation in individuals with albinism

Answer 61

changes to the DNA base sequence results in a change in the sequence of amino acids during translation a mutation in the gene coding for the enzyme could result in the active site of the enzyme changing shape the enzyme cannot function and no melanin is produced

Question 62

give an example of a gene mutation and what type of mutation it is (page 26-27 in booklet)

Answer 62

sickle-cell anaemia substitution

Question 63

sickle cell trait also confers some resistance to the malaria parasite why do you think this is that case?

Answer 63

plasmodium parasites cannot successfully reproduce within the red blood cells because of their sickle shape

Question 64

describe three possible changes in chromosome mutations

Answer 64

changes in chromosome structure: errors occur when chromosomes exchange sections of DNA during crossing over at prophase I changes in whole sets of chromosomes - polyploidy: if a gamete receives two sets of chromosomes during a failed meiotic division, the gamete will be diploid rather than haploid changes in chromosome number - nondisjunction: nondisjunction is a process in which faulty cell division results in one daughter cell getting two copies of a chromosome, whilst one daughter cell gets none

Question 65

how would fertilisation be affected if polyploidy occurs and where is polyploidy common in?

Answer 65

a triploid zygote (or tetraploid if two diploid gametes fuse) common in flowering plants such as tomatoes and wheat if it occurs in animals they will not survive and may be spontaneously aborted or stillborn

Question 66

what chromosome is affected in down's syndrome (trisomey 21) and describe how it happens?

Answer 66

chromosome 21 during meiosis in the female ovary, an oocyte with two copies of chromosome 21 is produced and survives if the oocyte is fertilised, the zygote would have 3 copies of chromosome 21 - two from the mother and one from the father

Question 67

give some features of down syndrome

Answer 67

open facial features low muscle tone some degree of learning disability

Question 68

explain how a trisomy such as trisomy 26 in mice can occur in a zygote

Answer 68

nondisjunction/homologous chromosomes fail to separate at anaphase I (in oogenesis) secondary oocyte has two copies of chromosome 16 third added from sperm at fertilisation

Question 69

define epigenetics

Answer 69

the control of gene expression by modifying DNA or histone proteins, without affecting the DNA nucleotide sequence

Question 70

describe epigenetics

Answer 70

evidence shows that the environment can alter the expression of genes by affecting how they are transcribed these changes are epigenetic - they affect gene expression but not their nucleotide sequence DNA can be modified post-replication this changes the ability of a gene to be transcribed in protein synthesis

Question 71

give two ways DNA can be modified post-replication

Answer 71

addition of a methyl group modification of histone proteins

Question 72

describe addition of a methyl group to DNA

Answer 72

a methyl group is added to nitrogenous bases this prevents these bases being recognised and therefore reduces the ability of the whole gene to be transcribed (expressed) if transcription does not occur translation will not occur, and the polypeptide and eventual protein will not be produced

Question 73

describe modification of histone proteins to DNA

Answer 73

these proteins are used to organise the DNA in a chromosome if the DNA becomes more tightly coiled around the histone proteins - this can prevent gene expression (transcription and translation) if coiled more loosely it can increase gene expression

Question 74

what results in different expressions of the same gene within one individual organism?

Answer 74

different epigenetic modifications can occur in cells of the same tissue and in different tissues of the same organism