U2T5 - Inheritance

Question 1

What does it mean if a gene has 2 alleles?

Answer 1

There are 3 possible combinations/genotypes that the organism may have. Dom/recessive homozygosity or heterozygosity.

Question 2

What did Mendel base his first law on?

Answer 2

Observations on outcome of monohybrid crosses from pea plants.

Question 3

Why is there only 1 gene for a characteristic in a gamete?

Answer 3

Meiosis 1 is reduction division + separates homologous chromosomes. Since each chromosome carries 1 gene at same locus in each, only 1 gene in each gamete.

Question 4

What is the ratio for:
2 hetero
1 homo + 1 hetero

Answer 4

3: 1
1: 1

Question 5

Why do 2 genes act independently of each other?

Answer 5

When pairs of homologous chromosomes arrange themselves on spindle equator during metaphase 1 of meiosis, they do so randomly. This random distribution + independent assortment produces new genetic combinations.

Question 6

What is an example of multiple alleles?

Answer 6

Human ABO blood groups. 3 possible alleles (A,B,O) represented normally by IA, IB, IO. A + B are codominant so in genotype IAIB, both expressed. Both dominant to O which is recessive so 4 poss blood groups.

Question 7

What are the 4 human blood groups and their possible genotypes?

Answer 7

A (IAIA, IAIO), B (IBIB, IBIO), AB (IAIB), O (IOIO)

Question 8

What are the male and female sex chromosomes?

Answer 8

M = XY - Heterogametic
F = XX - Homogametic

Question 9

Describe sex chromosome inheritance.

Answer 9

Homologous pairs of chromosomes come from each parent, half from mum + half from dad. All gametes contain 1 of 2 sec chromosomes which pair up at fertilisation. Inherit sex chromosome from dad, determines sex.

Question 10

What causes Down’s Syndrome?

Answer 10

Three copies of chromosome 21.

Question 11

Discuss how cats inherit coat colour.

Answer 11

Locus on x chromosome. Females have 2 alleles for coat colour + males only 1. Xo produces orange, Xb produces black. Codominant so both expressed in hetero, creates tortoiseshell cats (XoXb)

Question 12

Describe an example of polygenic inheritance.

Answer 12

Inheritance of seed colour in wheat. Determined by 3 genes on diff chromosomes. Each has 2 alleles. Dark red have genotype R1R1R2 R2R3R3. White is r1r1r2 r2r3r3. Colour determined by num of R alleles, not r alleles.

Question 13

Why are sex chromosomes usually carried on the X chromosome?

Answer 13

It’s larger so can carry many genes not present on Y chromosome, but y chromosome may carry some male gender characteristics.

Question 14

What does it mean if the phenotypic ratio is 1:2:1?

Answer 14

Codominance.

Question 15

Is it possible for parents of blood group B and A to produce any of 4 blood groups, especially O?

Answer 15

Yes as O can be produced from hetero A + B blood groups.

Question 16

Why are male cats never tortoiseshell in colour?

Answer 16

Can’t have codominance as Y doesn’t carry alleles. XoY rather than XoXb.

Question 17

What are 2 examples of sex-linked characteristics in humans?

Answer 17

Haemophilia + colour blindness. Carried on X chromosome. Inheritance of condition on X chromosome denoted with letter X + dom/rec allele as smaller letter on top right of X.

Question 18

Why are females less likely to suffer from haemophilia?

Answer 18

As they have 2 X chromosomes, dominant one can stop expression of recessive one so stopping haemophilia being expressed. Men only have 1 X chromosome so if it carries the allele for haemophilia, nothing can block it.

Question 19

How can environmental factors affect continuous variation?

Answer 19

E.g. height depends on both genotype and environment. Weight more influenced by environment (birth weight - well fed mum has bigger baby than poorly fed mum)

Question 20

Explain how mice inherit coat colours by epistasis.

Answer 20

Most have agouti colour (allele A, dom) black is a. White due to rec allele elsewhere so mice homozygous recessive (ww). If mouse homo rec, will be white irrespective of alleles at other locus. So, AAww, Aaww + aaww are all white. Must have 1 dom allele for each gene to be agouti (AAWW, AaWW, AAWw, AaWw) Black mice must have 1 dom W + 2 rec a alleles.

Question 21

What is the formula for the chi-squared test?

Answer 21

X2 = sum of (O-E)2/E
O = Observed nums obtained
E = Expected nums obtained
Use table when carrying out calcs.
Once X2 value obtained, probability table used to find out what it indicates.

Question 22

What is the critical value for probability tables and describe how the hypothesis is rejected or accepted?

Answer 22

5% (0.05).
If p over 0.05, hypothesis is supported so no signif diff.
If p under 0.05, hypothesis not supported + diff is signif.
Above 5% is still 3:1 ratio or 9:3:3:1 ratio. Slight deviation is due to chance.

Question 23

Why would the son of a haemophiliac man and healthy non carrier woman never inherit haemophilia?

Answer 23

Dad gives son Y chromosome so gets normal gene from mum’s X chromosome. Dad can’t pass on as not passing on X chromosome.

Question 24

Why is the colour of the seed coat determined by the number of R alleles rather than the number of r alleles?

Answer 24

r codes for white so only with at least 1 R can seed have colour. Colour depth depends on num of R alleles.