Topic 7: Genetics, Evolution and ecosystems

Question 1

What is a genotype?

Answer 1

The genetic makeup of an organism - all the alleles an organism has

Question 2

What is a phenotype?

Answer 2

• Observable characteristics of an organism

- The interaction between the expression of a dominant characteristic and the environment

Question 3

What is a gene?

Answer 3

A sequence of DNA that codes for a polypeptide or a functional RNA (tRNA/rRNA)

Question 4

What is an allele?

Answer 4

Different versions of the SAME GENE located at a locus on a chromosome

Question 5

How many alleles of each gene do diploid cells usually have?

Answer 5

2 (One is dominant, other recessive)

Except on the Y chromosome (much shorter)

Question 6

What does it mean when an organism is homozygous for a characteristic?

Answer 6

The allele is the same on both chromosome

Question 7

What does hetrozygous mean?

Answer 7

The alleles (of the same gene) are different on each chromosome

Question 8

What is a dominant allele?

Answer 8

An allele which is expressed all the time

Question 9

What is a recessive allele?

Answer 9

The allele is not expressed in a heterozygote (ie. with a dominant allele)

It is only expressed in a homozyous recessive pair of chromosomes

Question 10

What is it called when two alleles BOTH contribute to a phenotype?

Answer 10

Codominance

Question 11

What is an example of codominance?

Answer 11

The ABO blood group system
A and B are both dominant to the allele O

Eg AO = Blood group A
AB = Blood group AB
Blood group O only occurs with OO alleles as

Question 12

What is monohybrid inheritance?

Answer 12

The inheritance of a single gene

Question 13

Who investigated monohybrid inheritance?

Answer 13

Mendel, using pea plants because of easily observable characteristics

Question 14

When a tall plant is crossed with a short plant (both homozygous) what are the phenotypes of the F1 generation? (Tall is domonant)

Answer 14

Tall

Expalaination:
TT x tt

= Tt and Tt

Question 15

If two heterozygous tall (Tt) plants are crossed, what is the outcome?

Answer 15

3 Tall plants, 1 short plant
(TT, Tt x2, tt)

A 3:1 ratio

Question 16

What is the difference between observed and expected ratios is called?

Answer 16

A sampling error

Question 17

What effect does the sample size have on genetic sampling errors?

Answer 17

Small populations will have a greater impact (more sampling error)

Question 18

Why does a 9:3:3:1 or a 3:1 ratio rarely exactly exist (4 possibilities)?

Answer 18

A VERY COMMON GENETICS QUESTION

1. Fertilisation is random
2. Some alleles may be lethal
3. Some populations may be small, so a more pronounced sampling error
4. Some alleles may not be equally distributed among ‘viable gametes’, some die or are infertile

Question 19

How can you tell if a tall plant is homozygous or heterozygous for a tall allele? (in monohybrid inheritance)

Answer 19

• Self-fertile the tall plant
• Back-cross using a short plant
  Observe the ratio of offspring

Question 20

What do pedigree diagrams show?

Answer 20

Shows the ancestral relationship and transmission of genetic traits

Question 21

Why are pedigree diagrams useful?

Answer 21

• Detects genetic diseases

- Predicts the chances of offspring being affected by a genetic disease

Question 22

How can you tell form a pedigree diagram whether an allele is dominant or recessive?

Answer 22

• Look for parents that DO NOT have a characteristic but children who DO have it
• Tells you that the allele is recessive

Question 23

What does dihybrid inheritance look at?

Answer 23

Looks at how two different characteristics, determined by two different genes on different chromosomes, are inherited

Question 24

What is an example of dihybrid inheritance?

Answer 24

Seed shape and colour in pea plants

Seed shape and colour are determined by two different genes on two different chromosomes

Question 25

What ratio of phenotype is expected in a dihybrid cross of homozygous parents (eg RrYy)?

Answer 25

9:3:3:1

Question 26

Why does a dihybrid cross of homozygous parents (RrYy) produce 4 different types of gamete?

Answer 26

- The genes are on separate chromosomes - Homologous chromosomes arrange themselves on the cell equator at random in metaphase 1 of meiosis - Either Y and y or R and r can combine

Question 27

What are Mendel's two laws?

Answer 27

1. Law of segregation | 2. Law of independent assortment

Question 28

What is the law of segregation (Mendel)?

Answer 28

- Phenotypes are controlled by genes that occur in pairs | - Only one of each pair can be carried in the gamete

Question 29

What is the law of independent assortment (Mendel)?

Answer 29

Each member of a pair of alleles can combine randomly with either from another pair

Question 30

What is an autosome?

Answer 30

A chromosome that is not a sex chromosome (22 pairs in humans)

Question 31

What is autosomal linkage? What ratio of offspring would be expected?

Answer 31

Two or more genes are carried on the same autosome 3:1 ratio

Question 32

Do linked genes separate?

Answer 32

No, in accordance with Mendal's laws

Question 33

In what stage of meiosis does crossing over occur?

Answer 33

In prophase 1

Question 34

What is codominance?

Answer 34

When both alleles are expressed in a phenotype - neither is dominant over the other

Question 35

What are the alleles of the offspring of two parents with heterogeneous for codominant alleles?

Answer 35

Parents both have alleles RW Offspring have alleles RR, RW, RW, WW So a 1:1:2 ratio of phenotypes

Question 36

Give an example of codominace in humans. What are the alleles and phenotypes?

Answer 36

Sickle cell anaemia Normal haemoglobin governed by H allele, sickle cell governed by S allele Neither is dominant, so heterozygous individuals (HS) have sickle cell trait which causes partial dysfunction of haemoglobin

Question 37

What is Epistasis?

Answer 37

Where the allele for one gene affects or masks the expression of another in the phenotype

Question 38

What is an example of Epistasis? What genes are masked?

Answer 38

Coat colour in mice Gene A controls menalin DISTRIBUTION Dominant allele A leads to black bands Recessive allele a leads to uniform black Gene B determines the expression of A because: Dominant allele B leads to menalin production Recessive allele b leads to no menalin produced Thus, if Gene B is bb, then no pigment is expressed so no colour regardless of gene A expression (albedo)

Question 39

How many sex chromosomes do humans have?

Answer 39

A pair (2)

Question 40

What are the sex chromosomes in women?

Answer 40

XX

Question 41

What are the sex chromosomes in men?

Answer 41

XY

Question 42

What is special about the Y chromosome?

Answer 42

It is much shorter than the X chromosome

Question 43

Which sex chromosomes do male offspring inherit?

Answer 43

Y chromosome always from their father | Thus X chromosome is always one of their mothers

Question 44

What happens if a male offspring possesses a recessive allele on the non-homologous part of their X chromosome?

Answer 44

It is always expressed because the Y chromosome is much shorter, so there is no dominant allele

Question 45

What is the non-homologous part of an X chromosome (sex linkage)?

Answer 45

The portion of the X chromosome that has no equivalent gene (and thus allele) on the Y chromosome because the Y chromosome is much shorter

Question 46

The allele governing colour-blindness is sex linked. How can two 'normal' people produce colour-blind offspring?

Answer 46

The mother must be a carrier (as the recessive allele is on the X chromosome, which is always expressed in men because no equivalent dominant allele on the Y chromosome)

Question 47

Can females be colour-blind?

Answer 47

Yes (sex linked) Female parent must be carrier or colour blind and the father must be too Female offspring inherit on X chromosome form each parent

Question 48

What must you do when give the question: "What is the probability that the offspring will be male and _"?

Answer 48

Multiply the probability of being _ by the probability of being male (0.5)