Genetics

Question 1

Define locus

Answer 1

The position of a gene on a chromosome

Question 2

Why are some genes said to be linked?

Answer 2

Genes with loci on the same chromosome are linked; bc this means they’ll stay together during independent assortment + their alleles passed onto offspring together
- Closer together loci = more close linkage

Question 3

What is sex-linked characteristic?

Answer 3

When the locus of the allele coding for it is on a sex chromosome

Question 4

Why are males more likely to show recessive phenotypes for sex-linked genes?

Answer 4

• Y chromosome is smaller than the X + carries fewer genes
• as males have only one X chromosome, often have one allele for sex linked genes, meaning they express the alleles characteristic even if it’s recessive
• also more likely to express disorders caused by faulty genes on X chromosomes called X-linked disorders

Question 5

Define autosomal

Answer 5

Describes all chromosomes excluding sex chromosomes

Question 6

Define stem cell

Answer 6

An undifferentiated cell which can divide to become other cell types

Question 7

Define the three types of stem cells

Answer 7

Multipotent = can give rise to many different cell types
Pluripotent = able to give rise to many types of specialised cells but not placental cells
Totipotent = can give rise to all/any types of specialised cells including placental cells

Question 8

What are the two sources of stem cells and which type are they?

Answer 8

• Embryonic stem cells (totipotent) or adult stem cells (pluripotent)

Question 9

Give examples of sources for adult stem cells

Answer 9

• bone marrow
• skin
• brain
• umbilical cord blood

Question 10

What are the positives and negatives of stem cell use?

Answer 10

+ Can be used to treat diseases or replace damaged tissue, could improve quality of life of people
- Adult stem cells can have permission from donor
— Unethical - people may object to killing of embryos in embryonic stem cell extraction, often use IVF embryos
- Also a risk of infection when cells are transported

Question 11

Name the process by which cells become specialised

Answer 11

Differential gene expression

Question 12

Describe the process of cells becoming specialised

Answer 12

~ Stimulus acts on unspecialised cells
~ Activator + repressor molecules bind to promoter regions on DNA sequence
~ Some genes become switched on and are active whilst others are switched off
~ Active genes are transcribed to produce RNA
~ mRNA is then translated on ribosomes + used to produce proteins
~ Protein is able to change structure + function of cells

Question 13

What are operons?

Answer 13

Section of DNA including:
- group of structural genes transcribed together
- control elements —> promoter region (where RNA polymerase binds to) + operator region (where transcription factors bind)
- regulatory genes coding for repressor/activator proteins

Question 14

What are regulatory molecules + 3 types?

Answer 14

• Molecules affecting expression of operons
  ~ repressors = proteins which suppress transcription of gene in response to external stimulus
  ~ activators = proteins which increase transcription of genes in response to external stimulus
  ~ inducers = small molecules activating or repressing transcription depending on needs of cell + substrate availability

Question 15

Describe the lac operon and how it is involved in gene expression

Answer 15

• Operon containing genes necessary to acquire + process lactose from environment
• used in E. coli as it can use lactose for energy when glucose is low
• when glucose is absent but lactose present, catabolise activator protein CAP binds to operator to activate transcription
• if lactose is absent, repressor binds to operator to prevent transcription

Question 16

Define phenotype

Answer 16

Characteristics of an organism, resulted from the interaction of genes of organism with its environment

Question 17

Describe the two types of variation in phenotype

Answer 17

Continuous = When individuals in a population vary within a range e.g. height, mass
Discontinuous = When there are distinct categories e.g. sex, shoe size

Question 18

Which characteristics tend to fall under discontinuous and continuous variation?

Answer 18

Monogenic —> coded for by one gene; tend to show discontinuous variation (e.g. blood type)
Polygenic —> coded for nu multiple genes on different loci; tend to show continuous variation

Question 19

What are epigenetic modifications?

Answer 19

Changes in DNA which alter expression of genes but do not change the base sequence

Question 20

How does DNA methylation result in epigenetic changes?

Answer 20

• Methyl groups are added to DNA, modifying the function of the DNA
• Typically suppresses gene transcription (turns genes off), preventing cell from converting back into stem cell or another cell type
  -Can be removed through demethylation

Question 21

How does histone modification result in epigenetic changes?

Answer 21

• DNA wraps around proteins called histones
• when histones are tightly packed, proteins reading the gene cannot access DNA, so gene is turned off
• when loosely packed, genes turned on
• acetylation (addition of acetyl group) results in loose packing of nucleosomes, allowing transcription

Question 22

How does non coding RNA result in epigenetic changes?

Answer 22

Non coding RNA controls gene expression by attaching to coding RNA (creates proteins) to break it down so it cannot create proteins

Question 23

Define codominance

Answer 23

When both alleles are expressed in the phenotype; occurs when heterozygous for a trait