Topic 15: Genotypes And Phenotypes

Question 1

What determines the appearance of an organism?

Answer 1

Expression of genes and influence of environment on gene expression.

Question 2

What is the phenotype?

Answer 2

Expression of an organisms genotype in its structural, biochemical and physiological characteristics.

Question 3

What is the genotype?

Answer 3

Refers to both the double set of genetic instructions in a diploid organism and the genetic makeup of an organism at one particular gene locus.

Question 4

What is homozygous?

Answer 4

Both chromosomes contain same copies of allele for particular trait. Both dominant or both recessive Allen may be inherited.

Question 5

What is heterozygous

Answer 5

Organism inherits two different allés for particular trait. Dominant allele is expressed.

Question 6

What is hemizygous?

Answer 6

Genotype of human male relating to gene carried on X or Y chromosome, thus gene only comprises one allele.

Question 7

Talk about genes located on X chromosome.

Answer 7

• sex linked
• X chromo has more genes on it due to larger size than Y chromosome
• with gene located on sex chromosomes, traits controlled by alleles do not appear equally in both sexes
• females must inherit two copies of allele to be affected whereas males require one only thus males show x-linked traits more
• haemophilia, muscular dystrophy

Question 8

Talk about Y chromosomes.

Answer 8

Small segments of DNA at ends are homologous to those on ends of X chromosome, allowing sex chromosomes to synapse to form bivalents and then disjoin correctly in Meiosis as they behave as a homologous pair. X linked genes include amely (tooth enamel organisation), and A2F controlling sperm formation.

Question 9

What is complete dominance?

Answer 9

Occurs where mutually exclusive genetic instructions given, only 1 trait thus can be expressed.

Question 10

What is dominant?

Answer 10

Allele expressing phenotype in heterozygous and homozygous forms.

Question 11

What is recessive?

Answer 11

Trait that does not show phenotype unless in homozygous form.

Question 12

What is a carrier?

Answer 12

Heterozygote that has allele for recessive trait but does not express it.

Question 13

How do you decide whether a trait is dominant or recessive and why?

Answer 13

Phenotype of heterozygous organism must be identified. Trait that is expressed must be dominant, as dominant trait is expressed in hétérozygotes.

Question 14

What is co-dominance ?

Answer 14

Both alleles expressed in heterozygous condition. For example, blood type AB is codominant phenotypic expression of alleles A and B in heterozygous condition.

Question 15

Name environmental factors that impact the expression of genes.

Answer 15

Wind exposure, water availability, acidity, temperature, soil type.

Question 16

Describe the inherited disorder PKU (phenylketonuria).

Answer 16

• phe hydroxylase enzyme cannot be produced as baby inherited two recessive alleles thus recessive trait expressed
• offspring fed diet with protein including amino acid phe, baby suffers brain damage, becomes mentally retarded
• diet with low levels of phe amino acid, normal phenotype expressed

THUS phenotype due to particular genotype only appears in specific environment, different environment may result in another phenotype.

Question 17

What are monogenic traits?

Answer 17

• variation occurring in individuals due to action of different alleles of single gene.
• multiple alleles, trait expressed by single gene

Question 18

What is discontinuous variation?

Answer 18

Members of population classified into distinct non overlapping classes. Quantitative, described in words.
Monogenic traits show discontinuous variation.

Question 19

What are polygenic traits?

Answer 19

• Caused by action of two or more genes at different loci
• genes involved = polygenes
• often influenced by environmental factors.

Question 20

What is continuous variation?

Answer 20

Variety of phénotypes, cannot be categorised into distinct groups.
Quantitative values attained. Bell shaped distribution.
As number of polygones involved in expression of single trait increases, number of possible variations in trait also increases.

Question 21

Describe the four steps of NATURAL SELECTION

Answer 21

1. Variation exists in characteristics within species - original changes appear via mutations
2. Differential survival: selective pressure selects for favourable characteristics with survival advantage as they can survive better in environment.
3. Genetic inheritance: individuals with favourable characteristics survive and reproduce passing characteristics onto subsequent generations which in turn has more offspring
4. Frequency of organisms with advantageous characteristics increase in population thus change in population over time

Question 22

What is epigenetics?

Answer 22

All changes to genes aside from changes to base sequences which bring about phenotypic changes. Epigenetics includes factors that can bring about these changes acting on DNA to permanently turn it on or off.

Question 23

What are epigenetic factors?

Answer 23

Change how DNA in cells is packaged/labelled.

Genes in segments of DNA tightly packaged = silenced.
Genes in segments of DNA with opening packaging = active in transcribing polypeptide gene products.

Question 24

Name an example of an epigenetic tag.

Answer 24

• labelling of DNA doesn’t change base sequence BUT can silence or make active genes.
• methyl groups (-CH3) is epigenetic tag added to cytosine base
• active genes = fewer methyl groups
• thus addition of methyl gene changes gene expression by permanently switching genes off

Question 25

What happens to epigenetic tags once they have attached to base sequences

Answer 25

• Remain for life of the cell, transmitted to all daughter cells.
• Usually not passed to next gen as DNA of fertilised cell cleared of tags
• Epigenetic tags may however be not erased but conserved and passed to subsequent generations.

Question 26

Explain epigenetic inheritance in terms of cell differentiation.

Answer 26

• embryo cells derived from single fertilised egg via mitotic divisons
• cells develop along different pathways - brain cells, nerve cells etc
• identical génotypes present but different sets of genes active to differentiate
• epigenetic factors produce changes starting stem cells done different developmental paths

Question 27

Describe epigenetic inheritance in terms of X inactivation.

Answer 27

• somatic cells of normal female embryos 2 copies of X chromosome
• in early embryonic development, one X chromosome inactivated
• epigenetic tag causes inactivation which transmits to all sighted cells produced
• through mitotic cell divisons same X chromosome remains inactive

Question 28

Describe chemical action in terms of epigenetic inheritance

Answer 28

• ingestion of fungicide vinzoclozin interferes with sperm formation
• pregnant rats injected, male offspring produced reduced numbers of sperm with lower mobility
• defect passed to subsequent generations
• chemical did not mutate DNA, rather change in methylated DNA (increase) observed across 3 gens