Topic 14

Question 1

The appearance of an organism is the result of two forces…

Answer 1

1. The expression of the genes

2. The influence the environment has had on the expression of the genes.

Question 2

Phenotype

Answer 2

An organism’s expressed physical trait. The visible or measurable expression of the genetic make-up of an organism’s structure and/or function.

Question 3

Examples of phenotype

Answer 3

height, weight, hair colour, skin colour, colour of bird, ability to change colour

Question 4

Genotype

Answer 4

The underlying genetic make-up that determines an organism’s phenotype. The combination of the particular alleles of a gene that is present in a cell or organism.

Question 5

Homozygous

Answer 5

When both chromosomes contain same copies of the allele for a particular trait.

Question 6

Homozygous dominant

Answer 6

When both dominant alleles have been inherited

Question 7

Homozygous recessive

Answer 7

When both recessive alleles have been inherited

Question 8

Heterozygous

Answer 8

When an organism inherits two different alleles for a particular trait.
The dominant allele will be expressed

Question 9

Hemizygous

Answer 9

Refers to the genotype of the human male in relation to any gene carried on either the X or Y chromosome and which for each gene comprises only one allele.

Question 10

Autosomal genes

Answer 10

Both males and females carry to copies of each gene, one maternal one paternal. a gene that controls one function can exist in multiple forms or variants. Most genes only have two alleles, but some genes can have three or more alleles.

Question 11

Genes on the X chromosome

Answer 11

The X chromosome is larger than Y and so, it has many more gene loci.
The genes controlling colour vision and blood clotting are located on the X chromosome but not on the Y chromosome.
To be affected, females must inherit two copies of the particular allele while males are affected if they have just one allele so males more commonly show the trait.
When a gene is located on a sex chromosome, the traits controlled by its alleles do not appear equally in both sexes.

Question 12

Examples of X-linked genes

Answer 12

haemophilia, colour blindness, muscular dystrophy

Question 13

Genes on the Y chromosome

Answer 13

DNA segments at the ends of the Y chromosome are homologous with DNA segments on the X chromosome. During prophase of meiosis, homologous chromosomes synapse. Because regions of the Y chromosome are homologous to regions on the X chromosome, during meiosis in males the sex chromosomes can pair up and then disjoin correctly.
More than 95% of the Y chromosome is not shared and specific to males only. The DNA consists of 50 genes which control the production of proteins that produce male characteristics, such as testis formation.

Question 14

Examples of Y-linked genes

Answer 14

Hair loss (SOX21) , enamel (AMELY), sperm formation

Question 15

Dominant

Answer 15

An allele that expresses its phenotype in the heterozygous and homozygous forms

Question 16

Recessive

Answer 16

An allele that does not express its phenotype unless in the homozygous form

Question 17

Carrier

Answer 17

A heterozygote that has the allele for a recessive trait but does not show the trait

Question 18

Co-dominance

Answer 18

Both alleles are expressed in the phenotype in the heterozygous condition

Question 19

Human ABO blood system

Answer 19

Controlled by one gene each with three alleles
Alleles: Ia Ib I
Each responsible for producing a different version of the same gene
Ia and Ib are co-dominant

Question 20

Environmental effects on expression of genes

Answer 20

External environmental factors play an important role in modifying the phenotype coded by genes, both in the embryonic development and in later life.

Question 21

Environmental factors include such variables as…

Answer 21

Wind exposure
Water availability
Acidity
Temperature
Soil type
Light
Predation

Question 22

Example: environmental effects on expression of genes

Answer 22

Altitude can affect the growth of some plants
Large trees grow to full height of their genetic potential at base of hill. As altitude increases, there is severe stunting near the snowline.

Question 23

Monogenic traits

Answer 23

Variation that occurs in individuals of a species due to the action of different alleles of a particular gene.
When a gene has two alleles, dominant and recessive, there are only two variations in the phenotype of a population.
However, when there are three alleles, there are atleast 3 variations within a population.

Question 24

Discontinuous variation

Answer 24

When members of a population can be classified into a few discrete and non-overlapping classes.
Monogenic traits how discontinuous variation
Qualitative

Question 25

Polygenic traits

Answer 25

Variation that is caused by the action of two or more genes at different loci. The genes involved are called polygenes and their expression is also influenced by environmental factors.

Question 26

Continuous Variation

Answer 26

When members of a population cannot be classified into few distinct groups, they show a variety of phenotypes. Polygenic traits show continuous variation. Quantitative

Question 27

Example: polygenic traits

Answer 27

maximum speed of thoroughbred horses

Question 28

Environmental effects on species

Answer 28

Natural selection- causes species to change overtime In complex eukaryotic organisms, changes in species are slow and gradual due to slower reproductive rates In simple organisms, such as prokaryotic organisms, changes in species can be more rapid due to faster reproductive rates.

Question 29

Sequence of life

Answer 29

The first organisms were prokaryotes, then single celled eukaryotes, then multicellular eukaryotes

Question 30

In order for a species to evolve, a number of steps must be followed

Answer 30

1. There must be variation in characteristics within the species (original changes appear in species via mutations) 2. Differential survival- a selective pressure or selective agent selects for characteristics which best equip the individual to survive better in its environment with other members of the species with different characteristics. 3. Genetic inheritance- Individuals that have a characteristic that allows them to survive have a selective advantage. They will reproduce and pass on the genes for that characteristic to the next generation. 4. With time, as long as the selective agent is still active, the frequency of individuals with the advantageous characteristics will increase in the population.

Question 31

Epigenetics

Answer 31

All changes to genes, apart from changes to their base sequences, which bring about phenotypic changes. Epigenetic factors are factors that can bring about these changes, which act on DNA to turn the genes permanently on or off.

Question 32

Epigenetic factors

Answer 32

Can change how DNA in cells is packaged or how it is labelled. Genes in segments of DNA that are tightly packed- silenced with open packaging- active in transcribing polypeptide gene products

Question 33

Methylation of DNA

Answer 33

Methylation of DNA and histones causes nucleosomes to pack tightly together. Transcription factors cannot bind the DNA, and genes are not expressed.

Question 34

Histone Acetylation

Answer 34

Results in loose packaging of nucleosomes. | Transcription factors can bind the DNA and genes are expressed.

Question 35

Labelling DNA

Answer 35

Is like adding a tag that does not alter the base sequences of genes but can either silence genes or make them active.

Question 36

Methyl groups

Answer 36

An example of an epigenetic tag that can be added to any C base alongside a G base in DNA (-CH3)

Question 37

Tagging genes by the addition of methyl groups

Answer 37

Active genes are found to have fewer methyl groups than inactive genes, so it appears that tagging genes by the addition of methyl groups to their C-G bases can change gene expression and permanently switch those genes off.

Question 38

Inheritance of epigenetic tags

Answer 38

Once established, epigenetic tags remain for the life of a cell and are transmitted to all daughter cells derived from that cell. Usually epigenetic tags are not passed onto the next generation because the DNA of a fertilised egg is cleared of the epigenetic tags. In some cases, however, the epigenetic tags on the DNA are not erased but instead are conserved and passed to the next generation.

Question 39

Examples of epigenetic inheritance: cell differentiation

Answer 39

The cells of a human embryo all derive from a single fertilised egg by mitosis. During embryonic development, cells develop along different pathways. All the cell types have the same genotypes but different sets of genes are active in each cell type. Epigenetic factors produce the changes that start various stem cells down different developmental paths.

Question 40

Examples of epigenetic inheritance: X- inactivation

Answer 40

All female mammals have two copies of the X-chromosome. Early in embryonic development, one of the X-chromosomes in each somatic cell is inactivated, switching off all its genes. The epigenetic tags causing the inactivation are passed on to each daughter cell, through mitosis, so the same X chromosome remains inactive.

Question 41

Chemical action

Answer 41

Rats were injected with vinclozolin (commercial fungicide) which interferes with sperm formation. When pregnant rats were injected, male offspring produced reduced numbers of sperm with lower than normal mobility. If male rats managed to mate, their sons showed the same defect and this was then passed on to the next generation. The chemical did not cause mutations in the original male's DNA. All three generations did, however, have a change in the level of methylation of their DNA.

Question 42

Polyphenism

Answer 42

The expression of different phenotypes in a species due to environmental influences.

Question 43

Examples: polyphenism

Answer 43

Sex determination in reptiles | Changes in pigmentation in the wings of some butterfly species as the seasons change.

Question 44

Phenotypic plasticity

Answer 44

The amount of change in a phenotype due to environmental influences.

Question 45

Examples: phenotypic plasticity

Answer 45

Plants show a high amount of phenotypic plasticity because they are unable to move and must therefore adjust to a wide range of environmental changes throughout their lives.

Question 46

What are some sources of genetically induced variation

Answer 46

Alleles, sexual reproduction, single nucleotide variations, mutations, histone modification, dna methylation

Question 47

What are some sources of environmentally induced variation

Answer 47

Competition, pathogens, predators, physical environment, toxins, drugs, nutrition.