Inheritance

Question 1

Why did Mendel use peas?

Answer 1

• easy to grow
• can self-fertilise or cross fertilise
• flowers and fruit in the same year
• lots of seeds so next generations phenotype is statistically meaningful

Question 2

Why did Mendel use characteristics such as yellow or green (or tall and dwarf) seeds to observe differences when breeding peas?

Answer 2

• characteristic controlled by single gene
• genes on different chromosomes
• clear cut so easy to tell apart

Question 3

What is monohybrid inheritance?

Answer 3

Inheritance of a single gene

Question 4

What is a test cross (back cross)?

Answer 4

cross between an individual with the phenotype of the dominant characteristic but unknown genotype with an indiividual that is homozygous recessive for the gene in question (from the offspring you can work out the genotype of the dominant individual)

Question 5

What is codominance?

Answer 5

Both alleles of a gene are expressed, for example someone with the blood group AB has both A and B antigens on their RBC

Question 6

What is incomplete dominance?

Answer 6

The heterozygote phenotype is intermediate between that of the two homozygotes e.g. pink is a mix between the red and white genotype on flowers

Question 7

What is dihybrid inheritance?

Answer 7

Two different genes located on two different chromosomes

(always 9:3:3:1 if parents are heterozygous for both genes)

Question 8

Meaning of progeny?

Answer 8

descendents/offspring

(to calculate it, divide the total number of that genotype by the number of homozygous recessive individuals e.g. 9:3:3:1)

Question 9

What is a dihybrid test cross?

Answer 9

tests a genotype by crossing it with an individual that is homozygous recessive for both genes

Question 10

What is meant by sex linkage?

Answer 10

genes on the sex chromosomes not on the autosomes
- when deciding phenotypes state gender as well as whether they have the disease

Question 11

In autosomal linkage, what could explain a different ratio of phenotypes between offspring than that which is expected?

Answer 11

• crossing over at chiasmata can occur
• four type of gamete produced (but not in equal numbers)

Question 12

What is autosomal linkage?

Answer 12

• Two different genes located on the same chromosome.
• dominant alleles of each gene are linked
• recessive alleles of each gene are linked
• any other genotype e.g. a dominant with a recessive is a result of crossing over at metaphase 1

Question 13

What is a gene (point) mutation?

Answer 13

DNA isn’t accurately copied in S phase before cell division
- new codon may code for the same AA (code is degenerate) however if it doesn’t the structure and function of the protein may be effected
- e.b. sickle cell disease

Question 14

What is a chromosome mutation?

Answer 14

chromosomes can become broken or damaged
- could effect a large number of genes
- chromosome does not rejoin accurately at the corresponding position on its homologous partner

Question 15

What is an aneuploidy mutation?

Answer 15

Whole chromosomes or small number of chromosomes may be lost or added in a phenomenon called non-disjunction. (This is when chromosomes fail to separate to the poles of dividing cells at anaphase one or chromatids fail to separate at anaphase 2)
Offspring has an evene number of chromosomes so can’t form gamete in meiosis. For example Down Syndrome where person either has no or two copies of chromosome 21

Question 16

What is meant by polyploidy?

Answer 16

Having more than two sets of chromosomes. This is common in plants and might have advantageous effects e.g. disease resistance. A defect in the spindle at meiosis means the number of chromosomes may double if the cell fails to divides following the first nuclear division after fertilisation

Question 17

What is a carcinogen?

Answer 17

An agent that causes cancer

Question 18

What is a tumour suppressor gene?

Answer 18

It suppresses the cell cycle (stops mitosis) so if this is inactivated uncontrolled cell division may occur

Question 19

What is an oncogene?

Answer 19

It is the mutated version of a protooncogene, this promotes cell division. If an oncogene becomes permenantly activated there is uncontrolled cell division.

Question 20

What is a benign tumour?

Answer 20

• slower growing
• doesn’t invade neighbouring tissues
• remains localised

maligant is opposite of that. Metatasises= cell spread in blood stream

Question 21

What is epigenetics?

Answer 21

The control of gene expression by modifying DNA or histones but not by affeting the DNA nucleotide secquence

Question 22

What does every gene have before the first exon?

Answer 22

a sequence of bases called the promoter region. This promotes the transcription of DNA as it allows transcription factor to bind. As more methyl groups bind to this region it means the regulator protein can’t attach so there is no gene expression as the gene is not transcribed. e.g. RNA Polymerase unable to attach

Question 23

How are histones modified? How does this effect gene expression?

Answer 23

Addition of acetyl groups. As more histone groups are added the DNA is able to uncoil from around the histones. This makes the DNA more accessible so theres more transcription and gene expression increases.

Question 24

What is always the null hypothesis?

Answer 24

There is no difference between observed and expected results

Question 25

What significance level does biology work to

Answer 25

5%

Question 26

Why might some phenotypes be more common than expected and others be rare?

Answer 26

Common phenotypes are to do with autosomal linkage, the rare phenotypes result from crossing over (recombinants)

Question 27

What is the format for writing conclusion on X^2 test?

Answer 27

At ….. degrees of freedom and p=0.05, X2 value is less/more than critical value (state figure). This means less/more than 5% probability that the difference is due to chance so accept/reject the null hypothesis

Question 28

What is the alternative hypothesis?

Answer 28

There is a significant difference between observed and expected results (of….)

Question 29

What is endomitosis?

Answer 29

replication of chromosomes that is not followed by cytokinesis. If this happens in an early embryo, four sets of chromosomes are incoporated into the nuclear envolope and you produce tetraploid cells.

Question 30

Why is polypolidy more common in plants?

Answer 30

• they can reproduce asexually
• they are hermaphroditic so do not use chromosomes to determine sex

Question 31

What are the five types of point mutation?

Answer 31

• addition/ subtraction- base added/removed. This results in a frame shift mutation and all subsequent AA incorporated will be altered
• duplication- same base incorporated twice
• substitution- different base incorporated. If this is at a significant site on the protein molecule it may make a difference in activity. If an enzyme, the structure of the active site could be destroyed
• Inversion- adjacent bases on the same strand change position

Question 32

Which types of genetic crosses do you need a capital letter with a superscript to represent?

Answer 32

Sex linkage, incomplete dominance and codominance

Question 33

What is the SRY gene?

Answer 33

sex determining region on the y chromosome, it is able to switch on other genes genes on other chromosomes so male characteristics are expressed

Question 34

What is meant by linked genes?

Answer 34

Genes are on the same chromosome so do not segregate independently at meiosis.

Question 35

How do you recognise linked genes?

Answer 35

Numbers of progeny with different combinations of characteristics do not correspond with Medelian ratios for example 9:3:3:1 is a mendelian ration. Four genotypes but not in this ratio suggests the genes may be linked

Question 36

Why might medelian ratios not be produced in the next generation due to linked genes?

Answer 36

Crossing over is rare so the majority of gametes should still be the parental BB or bb. So the number of gametes with different genotypes is not equal so medelian ratios are not produced in the next generation

Question 37

What can determine sex?

Answer 37

• temperature eggs incubated at
• ploidy level (e.g. male bees are haploid)
• chromosome structure e.g. Male birds are ZZ but females are ZW
• Sequential Hermaphrodism- e.g. when dominant female clownfish dies, the dominant male changes sexes and takes her place.