Inheritance (SCR)

Question 1

Inheritance as the fusion of haploid gametes to give a diploid zygote.

Answer 1

• Plants, animals and other eukaryotes that reproduce sexually pass genes to offspring in gametes. This is the basis of inheritance
• Male and female gametes have the same haploid number of chromosomes so male and female parents make an equal genetic contribution to their offspring
• As gametes are produced from diploid body cells, meiosis is required to halve the chromosome number
• Diploid body cells have two copies of each autosomal gene (genes located on a non-sex linked chromosome) Only one of each gene is passed on to offspring in the gamete
• Fusion of male and female gametes doubles the chromosome number, so the zygote is diploid, as are all body cells subsequently produced by mitosis

Question 2

Why would you carry out genetic crosses in flowering plants?

Answer 2

Patterns of inheritance can be investigated by crossing different varieties of flowering plants such as peas. Pea plants produce both male and female gametes so can self-pollinate and therefore self-fertilise

Question 3

How do you carry out a genetic cross in flowering plants?

Answer 3

1. Cut off all the anthers from a flower on the plant that is intended to be the female parent. The anthers must be removed before they start to shed pollen, so the flower cannot self-pollinate.
2. Enclose the flower in a paper bag to prevent insects or wind from transferring pollen to it.
3. When the stigma of the flower is mature, transfer pollen to anthers on the intended male parent.
4. Wait for the pollen to male gametes germinate and the male gametes to be carried down to the ovary in a pollen tube, where they will fertilise egg cells inside the ovules, resulting eventually in seeds which can be harvested

Question 4

What are genotypes

Answer 4

“The genetic makeup of an organism”
Humans and other diploid organisms have two alleles of autosomal genes, one inherited from each parent. There could be two copies of one allele, or two different alleles. For example, for a gene with the alleles D and d, an individual could have DD, dd or Dd. Combinations of alleles such as these are known as genotypes.

Question 5

what are alleles?

Answer 5

Alleles are different versions of the same gene

Question 6

What is the phenotype?

Answer 6

The phenotype of an organism is its observable traits (characteristics). Phenotype includes structural traits such as whether hair is curly or straight and functional traits such as the ability to distinguish red and green colours.
Most phenotypic traits are due to the interaction between the genotype of an organism and the environment in which it exists, but there are some determined solely by genotype and some solely by environmental factors.

Question 7

which phenotypes are determined by the genotype only?

Answer 7

• eye colour-brown or blue/ grey
• haemophilia-blood slow to clot
• ability to smell B-ionone-an odorant in violets.

Question 8

which phenotypes are determined by genotype and the environment?

Answer 8

• height in humans
• autism- a personality trait
• diabetes- failure to regulate blood glucose concentration

Question 9

which phenotypes/traits are determined by the environment only?

Answer 9

• scars due to surgery or wounds
• river blindness- due to parasitic worms in the eye
• body art such as tattoos or piercings

Question 10

Who discovered that there were dominant and recessive alleles, and how?

Answer 10

Using pure-breeding varieties of pea plant, Gregor Mendel discovered a pattern of inheritance in which one allele of a gene is dominant and another allele is recessive.

Question 11

what is the dominant allele?

Answer 11

Dominant alleles determine the phenotype in both individuals that are homozygous for the dominant allele (DD, for example) and in individuals that are heterozygous with one dominant and one recessive allele (Dd, for example).

Question 12

what is the recessive allele?

Answer 12

Recessive alleles only determine the phenotype if an individual is homozygous with two recessive alleles (dd, for example. Mendel crossed two pure breeding varieties together that differed in a clear trait such as height (tall or dwarf). With each of the traits that Mendel tested, the Fl offspring all had the same phenotype as one of the two parents. For example offspring of a cross between tall and dwarf pea plants were all tall. When Mendel self-pollinated these Fl pea plants, the phenotype that was not seen in the Fl generation reappeared in 25% of the F2 pea plants.

Question 13

What is phenotypic plasticity?

Answer 13

Organisms can respond to their environment by varying their patterns of gene expression and therefore their traits.
This is a form of adaptation, but it is reversible because genes have only been switched on or off, not changed into new alleles. It is known as phenotypic plasticity and is particularly useful if the environment a population inhabits is variable. For example, a person with pale skin may become darker-skinned if there is an increase in exposure to sunlight. A change in gene expression results in increased synthesis of the black pigment melanin in the skin. If the sunlight stimulus diminishes, gene expression reverts to its former pattern and the skin gradually becomes paler again, as the melanin concentration reduces. In some cases, phenotypic changes in traits cannot be reversed during the lifetime of the individual, but can when offspring are produced.

Question 14

What is phenylkeronuria?

Answer 14

The genetic disease phenylketonuria (PKU) is due to a recessive allele of the gene that codes for the enzyme phenylalanine hydroxylase. This enzyme converts phenylalanine into tyrosine.
The PKU allele is recessive because a carrier with one
PKU allele can still produce functioning enzymes by expressing their normal allele. A person with two recessive PKU alleles does not produce functioning enzyme so phenylalanine accumulates in the body and there is tyrosine deficiency.
In excess, phenylalanine impairs brain development, leading to intellectual disability and mental disorders.
This can be prevented by screening for PKU at birth and giving affected children a diet low in phenylalanine.

Question 15

Explain multiple alleles using the ABO blood group

Answer 15

The ABO blood group system in a human is an example of multiple alleles. One gene determines the ABO blood group of a person. There are 3 alleles of the gene Iᴬ, Iᴮ and i (Iᴼ). There are 4 different blood groups: A, B, AB, O.

• Iᴬ is dominant over i so people with either genotype IᴬIᴬ or Iᴬi are in blood group A
• Iᴮ is dominant over i, so people with either genotype IᴮIᴮ or Iᴮi are in blood group B
• i is recessive to Iᴬ and Iᴮ, so only people with genotype ii are in blood group O
• Iᴬ and Iᴮ are co-dominant so people with Iᴬ Iᴮ are in blood group AB

Question 16

What is incomplete dominance?

Answer 16

Some pairs of alleles show incomplete dominance. The phenotype of a heterozygous individual is intermediate between the phenotypes of the two types of homozygotes. For example, if homozygous red-flowered plants are crossed with homozygous white-flowered plants, the heterozygous offspring all have pink flowers.

White flowers contain no red pigment. Pink flowers are intermediate because they contain some red pigment, but less than in a red flower. If two pink-flowered plants are crossed together, the ratio of flower colours in the offspring is 1 red: 2 pink: 1 white.

Question 17

Explain co-dominance with reference to ABO blood group system

Answer 17

Some pairs of alleles show co-dominance. Homozygous individuals have a dual phenotype that is different to those of either of the two types of homozygotes.

The Iᴬ and Iᴮ alleles in the ABO blood group system are an example because blood group AB is a dual phenotype and is not intermediate between group A and group B

• Iᴬ causes the production of a specific glycoprotein in the plasma membrane of red blood cells that acts as an antigen
• Iᴮ causes production of another glycoprotein in the plasma membrane that acts as a different antigen

Question 18

What are the two sex chromosomes?

Answer 18

Humans have 23 pairs of chromosomes in their body cells. Sex is determined by the 23rd pair of chromosomes. There are two types of sex chromosomes, X and Y.

• Females typically have two X chromosomes, so all female gametes have one X chromosome and all offspring inherit an X chromosome from their mother
• Males typically have one X and one Y chromosome, so male gametes either contain an X or a Y chromosome
• The sex of offspring is therefore determined by the sperm that fertilises the egg. A sperm with a Y chromosome makes the resulting child male, whereas an X-bearing sperm makes the child female

Question 19

what does the Y chromosome do?

Answer 19

The Y chromosome is small with only about 55 genes, many of which are unique to the Y chromosome and are not needed in females. One key gene on the Y chromosome causes gonads in a human embryo to develop into testes. This gene is the testis determining factor (TDF). The developing testes in an embryo start to secrete testosterone, which causes the development of other organs of the male reproductive system.

The gonads develop into ovaries in an embryo without a Y chromosome and therefore no TDF gene. The embryonic ovaries start to secrete oestradiol, causing the development of the female reproductive system.

Question 20

what does the X chromosome do?

Answer 20

The X chromosome is relatively large and has around 900 genes, many of which are essential in both males and females. All humans must therefore have at least one X chromosome. Because females have two copies of genes on the X chromosome and males only have one, the inheritance pattern differs in males and females.

Question 21

what are sex-linked genetic disorders?

Answer 21

Sex-linked genetic disorders are due to genes located on the X chromosome. Most sex-linked disorders are due to a recessive allele of the gene. Males only have one copy of genes on the X chromosome, so they have the disorder if this one copy is the recessive allele. Females are much less likely to be affected because as long as one of their two X chromosomes carries the dominant allele, they are unaffected.

Question 22

What is haemophilia?

Answer 22

Haemophilia is an example of this pattern of sex-linked inheritance. People with this disorder, usually males, either lack or have a defective form of factor VIII. This protein is a clotting factor that normally circulates in the blood. Cuts and other wounds bleed for much longer than normal in people with Haemophilia.
The gene for factor VIII is located on the X chromosome. The allele that causes haemophilia is recessive. The frequency of the allele is about 1 in 10,000. This is therefore the frequency of the disease in boys. Females only develop haemophilia in the very rare cases where both of their X chromosomes carry the recessive allele, but much more frequently they are carriers of the haemophilia allele.

Question 23

how do you show sex-linked inheritance on a Punnett square?

Answer 23

The pattern of sex-linked inheritance can be shown using Punnett grids. The alleles should always be shown as a superscript letter on the letter X to represent the X chromosome. The Y chromosome should also be shown although it does not carry an allele of the gene.

Question 24

what is Huntingtons disease?

Answer 24

The disease causes a wide range of symptoms and people aren’t all similarly affected. Movement problems include trouble with coordination, and involuntary jerking or twitching movements (chorea), and, as the disease progresses, patients have problems with swallowing. Cognitive and behavioural symptoms include difficulties in learning new information, depression, personality changes, mood disorders and hallucinations. The age of onset is variable.

Question 25

why does huntingtons disease occur?

Answer 25

This disease is caused by changes to the HTT gene, located on chromosome four (autosomal). HTT encodes for a protein called huntingtin. Although all functions of this protein aren’t known, it has an important role for neurons in the brain. HTT contains a CAG trinucleotide repeat. If this trinucleotide is repeated over 36 times, the protein encoded by the HTT gene becomes abnormally large. A large number of repeats is correlated with an earlier onset of Huntington’s disease; more than 60 repeats result in juvenile Huntington’s.

• Expansions of 27-35 are intermediate; a person with an allele this size (and one normal) will not develop Huntington’s disease, however there is a small chance that when producing gametes, the expansion will get bigger in these cells.

Men and women can inherit Huntington’s disease, which is inherited in an autosomal dominant pattern. This means that children of a person with Huntington’s disease have a 50% chance of inheriting a copy of the causative allele of the gene.

Question 26

what is cystic fibrosis?

Answer 26

It is an autosomal recessive disease.
CF is an inherited disease that affects 1 in 2500 live births in the UK. It is caused by inheritance of a faulty gene, which leads to either a deficiency or absence of a protein called the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR controls the movement of chloride and sodium ions across cell membranes. Since water follows these ions across cell membranes, CFTR is an important regulator of water movement and hydration of cell surfaces. CF does not occur unless two copies of the faulty gene are inherited. CF is a recessive genetic condition.

Question 27

what is polygenic inheritance?

Answer 27

one trait is governed by more than one gene

Question 28

what is discrete variation?

Answer 28

with discrete variation, every individual fits into one of a number of non-overlapping categories. For example, all humans are in a blood group. AB, A, B, or O. In most cases discrete variation is due to one or at most a few genes, without the environment having any influence.

Question 29

what is continuous variation?

Answer 29

with continuous variation any level of a variable is possible, between the extremes. Continuous variation is due to the environment only in most cases, or to genes in combination with the environment.

Question 30

what is segregation and independent assortment-meiosis

Answer 30

Segregation in genetics is the separation of chromosomes and the genes that they carry, during meiosis. For example, in an individual with the genotype Dd, the alleles D and d will segregate. If the individual has the genotype Ee for a second gene, segregation could result in any of the combinations DE, De, dE and de. Which combinations are produced depends on the movements of chromosomes in meiosis. Assuming that the genes are on different chromosomes, this will depend on which way bivalents are orientated in Metaphase I or Metaphase II of meiosis. The orientation of each bivalent is random and unaffected by how other bivalents are orientated. The probability of each combination of alleles is therefore equal. This is independent assortment how different genes independently separate from one another when reproductive cells develop

Question 31

who discovered independent assortment and segregation?

Answer 31

Genes that assort independently, because they are on different chromosomes, are unlinked. Segregation and independent assortment were discovered by Gregor Mendel, who performed dihybrid crosses and recorded the results.

Question 32

what is the expected ration from two heterozygous parents?

Answer 32

the predicted ratio of phenotypes is 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green. This is based on Mendel’s second law, which is the independent assortment of genes in dihybrid crosses.

Question 33

what is the expected ratio when you cross a heterozygous individual with a homozygous recessive individual

Answer 33

Mendel also crossed Heterozygous round yellow peas (RrYy) with homozygous wrinkled peas (rryy). The expected ratio in this case is 1:1:1:1, because the round yellow parents produce equal numbers of RY and Ry and rY and ry gametes and the wrinkled green peas produce only ry gametes.

Question 34

what is autosomal gene linkage?

Answer 34

Some pairs of genes do not follow Mendels second law (independent assortment) and the expected 9:3:3:1 or 1:1:1:1 ratio is not found in dihybrid crosses. Instead the parental combinations of alleles tend to be inherited together. This is called gene linkage. It is due to a pair of genes being located on the same chromosome, and are therefore not separated. In most cases, this is an autosome, so the inheritance pattern is autosomal gene linkage.

Question 35

why does linkage occur?

Answer 35

Linkage is rarely 100% because crossing over during meiosis between the linked genes can generate new combinations of alleles. Crossing over resulting in an exchange of DNA between chromatids. The generation of new combinations of alleles is recombination. Individuals that have a different combination of alleles or phenotypic traits from parents, due to crossing over, are recombinants.

Question 36

what happens if autosomal gene linkage occurs?

Answer 36

Because connected genes contradict the independent assortment law, they reduce genetic variation

Question 37

how do you find the recombination frequency?

Answer 37

A recombinant is an individual with a different combination of alleles from either parent. To find the recombination frequency between two genes, an individual heterozygous for both genes is crossed with an individual homozygous recessive for both genes.

Question 38

what is the outcome of the cross of a heterozygous individual with an individual homozygous recessive for both genes for unlinked genes?

Answer 38

The Punnett grid predicts the outcome of a cross between pea plants with round yellow seeds that were heterozygous and plants with wrinkled green seeds that were homozygous recessive.

When Mendel performed this cross his results were 55 round yellow, 51 round green, 49 wrinkled yellow and 52 wrinkled green. This is close to a 1:1:1:1 ratio. The round green and wrinkled yellow offspring are recombinants because they have a new combination of traits, The expected recombination frequency due to independent assortment of unlinked genes is 50%.

Question 39

what is the outcome of the cross of a heterozygous individual with an individual homozygous recessive for both genes for linked genes?

Answer 39

The following diagram shows a cross between pure-breeding spotted short-haired and unspotted long-haired rabbits. The F1 hybrid offspring were back-crossed to unspotted long-haired rabbits. The observed results in the F2 generation are far from a 1:1:1:1 ratio. There are more offspring with the parental combinations of alleles and traits, and fewer recombinants with new combinations of alleles and traits. This shows that the genes are linked.

Question 40

how do you calculate the recombination frequency?

Answer 40

recombination frequency= number of recombinant offspring/ total offspring *100

Question 41

how do you carry out the chi-squared test?

Answer 41

1. Draw a table of observed frequencies
2. Calculate the expected frequencies, based on a Mendelian ratio and the total number of offspring
3. Determine the number of degrees of freedom (df), which is one less than the total number of possible phenotypes. In a dihybrid cross, there are four phenotypes so df=3
4. Find the critical value for chi-squared from a table of chi-squared values, using a df and significance level (p) of 0.05 (5%). The critical region is any value of chi-squared larger than the value in the table
5. calculate chi-squared using the equation:
   x^2=Σ (observed value-expected value)^2/(expected value)
6. compare the calculated value of chi-squared with the critical region. If the calculated value is greater than the critical value then the probability is less than 5% that the differences in the frequency are due to chance → reject null hypothesis .

Question 42

How does sickle cell anaemia work

Answer 42

Autosomal recessive
- - Mutation in the gene that codes for the alpha-globin polypeptide in haemoglobin
- The symbol for the gene is Hb
- Most humans have Hb^A
- In sickle cell anaemic individuals- the sixth codon in the alpha-globin gene changes from GAG to GTG
- This change forms the Hb^S form of the polypeptide
- This mutation is only inherited if it occurs within a sex cell
- During protein synthesis (transcription phase)- due to the mutation the 6th amino acid will be valine instead of glutamic acid
- This change causes the haemoglobin molecules to become sticky in low oxygen concentrations
- This causes the haemoglobin to become rigid and causes the haemoglobin molecule to form bundles within a red blood cell
- This causes the red blood cells to form sickle shapes
- Sickle cells can become trapped in capillaries→ reducing blood flow to tissues
- The bundles of haemoglobin in red blood cells break up in high oxygen concentrations and the cells can return to their normal shape
- When this occurs it damages the plasma membranes of the red blood cells and shortens the lifespan of the red blood cells (4 days), when they are normally 60-90 days
- The body cannot replace the red blood cells fast enough so anaemia develops.