Inheritance👨‍👩‍👧‍👦

Question 1

What is a gene?

Answer 1

A specific length of DNA which occupies a position on a chromosome called a locus

Question 2

What are polygenic traits?

Answer 2

Most characteristics are determined by several different genes interacting

Question 3

What is a locus?

Answer 3

A particular position of a gene on a chromosome

Question 4

What are alleles?

Answer 4

• Alternative forms of the same gene

* May alter the way in which a particular characteristic is expressed

Question 5

What is a dominant allele?

Answer 5

If this allele is present, the individual has the characteristic

Question 6

What is a recessive allele?

Answer 6

•Individual only has the characteristic if both chromosomes in the homologous pair have the recessive allele
-i.e. if the individual is homozygous recessive
•Cystic fibrosis is an example of a genetic condition that is caused by a recessive allele

Question 7

What does homozygous mean?

Answer 7

•Both chromosomes in a homologous pair have the same allele

Question 8

What is homozygous dominant?

Answer 8

Both chromosomes have the dominant allele

Question 9

What is homozygous recessive?

Answer 9

Both chromosomes have the recessive allele

Question 10

What does heterozygous mean?

Answer 10

• The alleles on the chromosome of the homologous pair are different
• Individual has a dominant and a recessive allele for a particular characteristic and could be said to be ‘carrying’ the recessive characteristic

Question 11

What does carrying mean?

Answer 11

Characteristic is not shown by the individual but could be passed on to their offspring

Question 12

What is a genotype?

Answer 12

The alleles possessed by an organism

Question 13

What is a phenotype?

Answer 13

The physical characteristics of an organism controlled by the genotype

Question 14

What is monohybrid inheritance?

Answer 14

• The inheritance of a pair of contrasting characteristics controlled by two alleles at a single locus
• An example is Gregor Mendel’s pea shoot experiments

Question 15

Step-to-step of a genetic cross

Answer 15

• Show genotypes of the parents
• Represent the possible gametes from each parent and draw a circle around them
• Construct a Punnett square and put the possible gametes from one parent in the first vertical column and the possible gametes from the other parent in the first horizontal row
• Carry out the genetic cross, combining the gametes, and write the genotype formed in the appropriate square
• Count the different genotypes and phenotypes and represent them as a percentage or a ratio

Question 16

What is a test cross/back cross?

Answer 16

• Used to work out whether an individual with a dominant characteristic is homozygous dominant or heterozygous
• Involves breeding the individual of unknown genotype with an individual that is homozygous recessive
• By looking at the phenotypes of the offspring, it is then possible to determine whether the unknown individual was homozygous dominant or heterozygous
• If breeding the individual of unknown genotype with a homozygous recessive individual produces any offspring expressing the recessive allele, the unknown individual must be heretozygous

Question 17

Expected phenotypic ratios in offspring

Answer 17

Crossing two heterozygous you will always get the ratio 3:1

Question 18

What is codominance?

Answer 18

• Some alleles are codominant

* Both alleles are expressed in the phenotype

Question 19

Example of codominance - blood groups in humans

Answer 19

• I^AI^A only have A antigens in their blood, while humans who are I^BI^B only have B antigens
• Heterozygous humans (I^AI^B) have both antigens in their blood as both alleles are expressed
• This means if one parent is I^AI^A and the other parent is I^BI^B all their offspring will be I^AI^B

Question 20

What are heterosomes?

Answer 20

• Human sex is determined by a pair of sex chromosomes called X and Y
• Called heterosomes as they don’t look alike

Question 21

What are autosomes?

Answer 21

All other chromosomes (other than heterosomes) are called autosomes

Question 22

What is sex-linkage?

Answer 22

• Genes that are located on one or other of the sex chromosomes are said to be sex linked
• A sex linked condition is one that is controlled by an allele on the X or Y chromosome

Question 23

Why are most sex linked conditions carried on the X chromosome?

Answer 23

Due to the small size of the Y chromosome

Question 24

Examples of sex linked conditions

Answer 24

• Haemophilia

* Duchenne muscular dystrophy

Question 25

What is haemophilia? (Sex linked genetic condition)

Answer 25

* Caused by a recessive allele found on X chromosome, represented by X^h * As on X chromosome, female haemophiliac must be homozygous recessive (X^hX^h) while a Male haemophiliac has only one recessive allele (X^hY)

Question 26

Why is it possible for a female carrier and a healthy male to have a haemophiliac son but not a haemophiliac daughter?

Answer 26

* X chromosome from father would be X^H as the father is not a haemophiliac * The mother of a female haemophiliac would have to be either a carrier or have the condition -her father would have to be a haemophiliac

Question 27

What is dihybrid inheritance?

Answer 27

* Involves the inheritance of two genes not carried on the same chromosome * Each gene has 2 alleles * Mendel studied this in seeds of pea plants

Question 28

What leads to different genotypes in the offspring of dihybrid inheritance?

Answer 28

Independent assortment

Question 29

What is the typical expected phenotype ratio in dihybrid inheritance?

Answer 29

9:3:3:1

Question 30

What is linkage?

Answer 30

•Alleles on the same chromosome will be inherited together •This is because when the chromosome passes into a gamete, the alleles are all present •Therefore, if that gamete is fertilised, all those alleles will be incorporated into the genome of the offspring •Do not follow the expected Mendelian ratios -non-Mendelian patterns of inheritance

Question 31

How is linkage further complicated?

Answer 31

* Crossing over swaps genes between chromosomes | * This means the linkage can be changed

Question 32

What are recombinant offspring?

Answer 32

Individuals with different phenotypes to parents

Question 33

Recombinant offspring in linkage

Answer 33

•Occurrence of recombinant offspring shows that linkage was not complete due to crossing over during prophase 1 of meiosis separating the genes

Question 34

Process of recombinant formation in linkage

Answer 34

* Homologous chromosomes pair up during prophase 1 * Chiasma formation * Homologous chromosomes separate during anaphase 1 * Chromatids separate during anaphase 2, forming 4 different gametes

Question 35

Down’s syndrome

Answer 35

Caused by an extra chromosome 21 - trisomy 21

Question 36

How can trisomy be achieved in a zygote?

Answer 36

* Due to non-disjunction in anaphase 1 of meiosis | * Chromosomes fail to separate

Question 37

Why was Gregor Mendel’s research useful?

Answer 37

* Large numbers of data so statistically valid * Meticulous records * Analysed data - conclusions made based upon evidence

Question 38

Particulate inheritance

Answer 38

Mendel thought characteristics were passed on by particles

Question 39

What were Mendel’s laws?

Answer 39

* Law of segregation | * Law of independent assortment

Question 40

The law of segregation?

Answer 40

* Every organism possesses a pair of alleles for any particular characteristic * Each parent passes a copy of only one of those alleles to their offspring * Whichever of the alleles is dominant affects the phenotype of the organism

Question 41

The law of independent assortment

Answer 41

* States that genes are passed independently of other genes from parents to offspring (the selection of a gene is not affected by the selection of other genes) * The genes assort independently during gamete formation

Question 42

Multiple alleles

Answer 42

More than two alleles

Question 43

Incomplete linkage

Answer 43

* Genes are on the same chromosome but are far apart | * Crossing over may occur between them during prophase 1

Question 44

Controlling gene expression

Answer 44

* DNA can be modified post-replication * This does not change the DNA base sequence but changes the ability of a gene to be transcribed during protein synthesis * The gene may be suppressed or expressed more

Question 45

Epigenetics

Answer 45

The control of gene expression by factors other than changes in the DNA sequence

Question 46

Epigenetic mechanisms

Answer 46

•DNA methylation - addition of methyl groups - prevents those bases being recognised and reduces the ability of that gene to be expressed •Histone acetylation - histone proteins used to organise the DNA in a chromosome can also be modified -If the histone is more tightly coiled, this can prevent gene expression -If the histone is less tightly coiled, this can increase gene expression •Different epigenetic modifications across different cells and tissues result in the expressions of the same gene in different parts of an organism

Question 47

Duchenne muscular dystrophy

Answer 47

* An example of a condition that is sex linked * The faulty gene is only carried on the X chromosome, so muscular dystrophy has the same inheritance pattern as haemophilia

Question 48

What are mutations?

Answer 48

* Random change in the volume, arrangement or structure in the DNA of an organism * Mutations can either affect a particular gene or a whole chromosome * Most mutations occur during crossing over in prophase 1 and through non-disjunction in anaphase 1 and anaphase 2

Question 49

What is addition? (gene mutations)

Answer 49

Nucleotides are inserted into the DNA sequence

Question 50

What is deletion? (gene mutations)

Answer 50

Nucleotides are removed from a DNA sequence

Question 51

What is substitution? (gene mutations)

Answer 51

A section of nucleotides is swapped for other nucleotides

Question 52

What is inversion? (gene mutations)

Answer 52

A section of nucleotides is reversed

Question 53

What is duplication? (gene mutations)

Answer 53

A section of nucleotides is duplicated

Question 54

What could a mutation result in?

Answer 54

* Produce a change in DNA sequence of a gene * During transcription, this change will be incorporated into the mRNA molecule formed * Could lead to the polypeptide formed in translation having an incorrect amino acid sequence - primary structure would be incorrect * Could lead to a change in the folding of the polypeptide and therefore to a change in the tertiary structure of the protein * An alteration in the tertiary structure of a protein could render it non-functional

Question 55

Sickle cell anaemia - gene mutation

Answer 55

* Mutation occurs in one nucleotide (T replaced by A) in one gene * In sickle cell anaemia, the RBC have a rigid sickle shape * Change in shape and lack of flexibility can lead to a variety of complications, including blockage of capillaries, which cuts off the blood supply to organs and results in organ damage

Question 56

Sickle cell anaemia - genetic terminology

Answer 56

•Allele that causes sickle cell anaemia is recessive - s •Therefore, only individuals who are homozygous for this allele have the condition •Heterozygous individuals have what is known as sickle cell trait, which doesn’t usually cause any symptoms -however, makes individual resistant to malaria -encourages sickle cell allele to be passed onto areas where malaria is prevalent as the heterozygous phenotype has a selective advantage

Question 57

Change in structure (chromosome mutations)

Answer 57

The structure of a chromosome can alter because of errors during crossing over

Question 58

Changes in number (chromosome mutations)

Answer 58

•Errors in meiosis can result in a gamete receiving the incorrect number of chromosomes •The embryo formed from this gamete will then also have an incorrect number of chromosomes •E.g. trisomy 21 -extra copy of chromosome 21 -Down syndrome

Question 59

Changes in sets of chromosomes (chromosome mutations)

Answer 59

* A gamete can have a whole extra set of chromosomes | * This is known as polyploidy

Question 60

Mutation frequency

Answer 60

It is important to remember that mutagens don’t always cause mutations - they increase the frequency of mutation

Question 61

Mutation rates

Answer 61

•Normally very low •In general, organisms with short life cycles and more frequent meiosis show a greater rate of mutation •Mutagens increase the chance of a mutation occurring -e.g. ionising radiations (UV light and X-rays) and certain chemical (polycyclic hydrocarbons in cigarette smoke) •A mutagen that increases chance of developing cancer is a carcinogen •Mutations in proto-oncogenes can result in them becoming oncogenes, leading to cells dividing uncontrollably and the development of cancer

Question 62

Explain why genes found in the sex chromosome pair have a pattern of inheritance that is different from genes found on other chromosomes pairs

Answer 62

* X chromosome carries genes/loci not present on the Y chromosome * Males only have one copy/allele of some genes * If only one allele inherited it will be expressed