Genetics

Question 1

Gene : Definition

Answer 1

A gene is a length of DNA that contains the coded instructions for building a gene product. The sequence of nucleotides, red in triplets, determines the order of amino acid monomers in the polypeptide.

Question 2

Chromosomes : Definition

Answer 2

The genome of a cell is contained in a number of separate, very long molecules of DNA. Each molecule of DNA, together with the histones around which it is wrapped, is called a chromosome.

Question 3

Karyotype : Definition

Answer 3

A karyotype is a photograph of a person’s chromosomes. Karyotype can be used to determine a number of things such as species, sex and some genetic abnormalities.

Question 4

Genotypes : Definition

Answer 4

Genetic constitution of a cell or individuals (what alleles it has for its genes) : aa / aa

Question 5

Phenotypes : Definition

Answer 5

Total appearance of an organism, determined by interaction during development between its genetic constitution and the environment : Purple leaves / Smooth edge

Question 6

Homozygous Genotype : Definition

Answer 6

The homozygous when referring to a diploid organism means that it has inherited the same allele from both parents : The genotype TT is homozygous dominant because there are two of the same dominant alleles together.

Question 7

Heterozygous Genotype : Definition

Answer 7

Heterozygous genotypes are organisms that have inherited different alleles from the parents : Tt is heterozygous as there are different alleles at the same stie.

Question 8

Complete Dominance

Answer 8

Example, in pea plants the gene for height has 2 different alleles. Y is for tall phenotype and t is for short phenotype : T = Tall plants, t = dwarf plants

Question 9

Dominant

Answer 9

A character is said to be dominant if it is the only character to show up in the offspring of a cross between two purbredding strains of a species.
Phenotype: Tall Plants x Short Plants
Genotype: TT x tt

Question 10

Recessive:

Answer 10

Recessive characters will only show up if two recessive alleles come together.
Phenotype : Tall Plants x Tall plants
Genotype: Tt x Tt

Question 11

Incomplete Dominance

Answer 11

Heterozygous individuals show a ‘blend’ of the characteristics
The heterozygous individuals look different from either of the homozygous individuals e.g. Red flowers x White Flowers → Offspring are all Pink
Capital letters are used for each allele or superscripts can be used e.g. Red = R and White = W : RR x W

Genotype Ratio 4RW : 0RR : 0WW
Phenotypic Ratio 4 Pink : 0 Red : 0 Blue

Question 12

Co-Dominance

Answer 12

In codominance heterozygous individuals both characteristics are equally observed, not a blend. E.g. a purple flower x a white flower → Offspring are purple and white striped flowers

Question 13

Environmental Variations:

Answer 13

Although identical twins have identical genotypes, their phenotypes can differ. E.g. Weight is an environmental factor, such as nutrition may be the cause. An example is the flower colors of hydrangea plants
- In acid soils, blue flowers are produced
- In alkaline soils, pink flowers are produced.

Question 14

Internal Environment : Definition and Example

Answer 14

Levels and types of hormone difference may occur between sexes like male lyrebirds having spectacular tail plumage whereas females do not.

This is evident in humans through Fetal Alcohol Syndrome. This is caused by the internal issue of excess consumption of alcohol during pregnancy.

Question 15

External Environment: Definition

Answer 15

Some animals’ fur colour is influenced by temperature. Himalayan rabbits, such as guinea pigs, some mice and siamese cats have chocolate points. These animals have the same genotypes in all cells but only some areas of the body - the extremities show the darker colouring. It has been suggested the gene is active (turned on) at lower temperatures or the enzymes that control this colouring are only active at low temperatures.

Question 16

Polygenic Inheritance:

Answer 16

Is where genes are working together to produce a particular trait. Some examples include human height and human skin colour. Human skin colour is thought to be controlled by at least three genes which interact to produce melanin as seen in the following punnett square.

Question 17

Epigenetics: Definition

Answer 17

Epigenetics is the study of changes in the organisms caused by modification of gene expression (active versus inactive genes) rather than alteration of the genetic code. So, it is a change in phenotype without a change in the genotype.

Question 18

Two main epigenetic mechanisms controlling gene expression …

Answer 18

1. DNA Methylation, which chemically modifies the base C, altering how the genetic code is read
2. Chromatin Remodeling, which alters the availability of genes for reading.

Question 19

Examples of Epigenetic Inheritance …

Answer 19

1. Agouti Viable Yellow (Ayy) Gene
2. Nurturing Behaviour in Rats
3. Water flea (Daphnia) protective helmets
4. Gestational Diabetes in mammals

Question 20

Agouti Viable Yellow (Ayy) Gene

Answer 20

The Ayy locus is actually a retrotransposon (i.e jumping gene) that is inserted upstream of the agouti gene. Ayy is unique to the mouse genome. Normally these Ayy elements are methylated, thus, shut off. However, in the Agouti mouse, they are unmethylated and active, leading to a yellow coat and very pronounced belly.
Agouti mice and regular mice both have the same genotype. However, they have a different epigenetic methylation pattern at a specific gene loci that impacts hair colour and weight.

Question 21

Nurturing Behaviour in Rats

Answer 21

Rat pups who receive high nurturing (high quality care) or low nuritn (low quality care) from their mothers develop epigenetic differences that affect their response to stress later in life. When the female pups become mothers themselves, the ones that receive high quality care become high nurturing mothers, and vice versa. The nurturing behaviour itself transmits epigenetic information onto the pups DNA, without passing through egg or sperm.

Question 22

Water flea (Daphnia) protective helmets

Answer 22

Female water fleas respond to chemical signals from their predators by growing protective helmets. The offspring of helmeted water fleas are also born with helmets - even in the absence of predator signals. This effect continues to the next generation, though the helmets in the grandchildren are much smaller.

Question 23

Gestational Diabetes in mammals

Answer 23

Mammals can experience a hormone-triggered type of diabetes during pregnancy, known as gestational diabetes. When the mother has gestational diabetes, the developing fetus is exposed to high levels of the sugar glucose. High glucose levels trigger epigenetic changes in the daughter’s DNA, increasing the likelihood that she will develop gestational diabetes herself.

Question 24

Monohybrid Cross: Definition

Answer 24

These are crosses which consider only one characteristic at a time e.g.g pea plant height, flower colour, each of these characteristics are controlled by one gene pair.

Question 25

Dihybrid Cross: Definition

Answer 25

When two characteristics are considered in the same cross it is called dihybrid cross.

Question 26

Test Cross

Answer 26

If an individual has a dominant phenotype but we do not know their genotype, we can work it out by doing a test cross. A test cross is crossing whatever you have to a homozygous recessive. A test cross is a type of backcross. A backcross is crossing the F1 with either one of the purebredding homozygous parental genotypes.

Question 27

Linked Genes

Answer 27

When two or more genes are found on the same chromosome they are said to be linked. This linkage can affect the outcomes of meiosis, because during meiosis crossing over can occur between homologous chromosomes. The crossing over may result in different combinations of alleles for each of the linked genes. Crossing over is a relatively common occurrence during meiosis however the closer two genes are on chromosomes the less likely that a crossing over event (chisma) will occur between them.

Question 28

Crossing Over and Recombination

Answer 28

During crossing over homologous chromosomes, they exchange or swap parts of their chromatids during the prophase 1 stage of meiosis. Crossing over between genes on homologous chromosomes can lead to new allele combinations. When crossing over occurs the phenotypic ratios of the offspring are different from those predicted from pure linkage. This is due to the new combinations of gametes. The further apart on the chromosomes the two gene loci are the greater the change of recombination.

Question 29

Sex Chromosomes and Inheritance

Answer 29

If the characteristic/trait is carried on the sex chromosomes it is called sex link. If it is on the X chromosome it is X-linked. Corresponding on the Y chromosome it is Y-Linked.

Question 30

Pedigree Analysis:

Answer 30

The offspring of any two parents are called collectively a sibship, and any offspring is a sib of any other. There are five different modes of inheritance examined in year 11: Autosomal Dominant Autosomal Recessive X-Linked Dominant X-Linked Recessive Y linked

Question 31

Autosomal Dominant Pattern:

Answer 31

1. Both males and females can be affected 2. All affected individuals have at least one affected parent 3. Transmission can be from fathers to daughters and sons, or from mothers to daughters and sons 4. Once the trait disappears from a branch of the pedigree, it does not reappear 5. In a large sample, approximately equal numbers of each sex will be affected

Question 32

Autosomal Recessive Pattern:

Answer 32

1. Both males and females can be affected 2. Two unaffected parents can have an affected child 3. All the children of two persons with the condition must also show the condition 4. The trait may disappear from a branch of the pedigree, but reappear in later generations 5. Over a large number of pedigrees, there are approximately equal numbers of affected males and females.

Question 33

X-Linked Dominant Pattern:

Answer 33

1. A male with the trait passes it on to all his daughters and none of his sons 2. A female with the trait may pass it on to both her daughters and her sons 3. Every affected person has at least one parent with the trait 4. If the trait disappears from a branch of the pedigree, it does not reappear 5. Over a large number of pedigrees, there are more affected females than males.

Question 34

X-Linked Recessive Pattern:

Answer 34

1. All of the sons of a female with the trait are affected 2. All the daughters of a male with the trait will be carriers of the trait and will not show the trait; the trait can appear in their sons 3. None of the sons of a male with the trait and an unaffected females will show the trait unless the mother is a carrier 4. All children of two individuals with the trait will show the trait 5. In a large sample, more males than females show the trait.

Question 35

Y-Linked Pattern:

Answer 35

1. Traits on the Y chromosome are only found in males, never females 2. The father’s trait is passed on to all sons 3. Dominance is irrelevant as there is only one chromosomes so only one copy of any Y linked gene (hemizygous)

Question 36

Aneuploidy

Answer 36

Missing or gaining an extra chromosome, in which often leads to physical or intellectual deficiencies.

Question 37

Gene Loci

Answer 37

A specific, fixed position on a chromosome where a particular gene or genetic marker is located

Question 38

Homologous Pairs : Similarities and Differences

Answer 38

Sim: 1. Same centromere placement along their lengths 2. Carry the same gene loci 3. Same genetic bands when stained 4. Have the same length Dif: 1. Some of the genes have different alleles

Question 39

Bird Karyotype : Opposed to Human

Answer 39

Bird Karyotypes are visibly different as bird species have W and Z sex chromosomes rather then X and Y like humans

Question 40

Bacterial Karyotypes : Opposed to Human

Answer 40

Bacteria karyotypes are visibly different as bacteria organism contain a single chromosome, opposed to humans who have 22 chromosome pairs.

Question 41

What shape represents which gender on a pedigree?

Answer 41

Square: Male Circle: Female

Question 42

What does Autosomal mean in terms of Autosomal Recessive Diseases?

Answer 42

Autosomal means that this genetic disease is located on a chromosome rather then a sex chromosome.