topic 3.4- inheritance

Question 1

how did Mendel discover the principles of inheritance?

Answer 1

with experiments in which large numbers of pea plants were crossed

Question 2

what did Mendel discover?

Answer 2

When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants

When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1
E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1

Question 3

gametes are haploid so…

Answer 3

contain one allele of each gene, and one chromosome of each type

Question 4

what is the name of the single cell produced when two gametes fuse?

Answer 4

a zygote

Question 5

define segregation of alleles

Answer 5

the separation of alleles into different nuclei

Question 6

define a dominant allele

Answer 6

an allele that masks the effects of a recessive allele

Question 7

define co-dominant alleles

Answer 7

alleles that have joint effects

Question 8

what is the usual reason for dominance of one allele?

Answer 8

this allele codes for a protein that is active and carries out a function, whereas the recessive allele codes for a non-functional protein

Question 9

what is a genetic disease and what is it caused by?

Answer 9

a genetic disease is an illness that is caused by a gene, and many genetic diseases in humans are due to recessive alleles of autosomal genes

Question 10

what is a carrier of a genetic disease?

Answer 10

an individual that only has one recessive allele for a genetic disease and one dominant allele, who therefore does not shows symptoms of the disease but can pass on the recessive allele to their offspring.

Question 11

give 2 causes of genetic diseases other than by recessive alleles

Answer 11

• dominant/co-dominant alleles
• sex-linked diseases

Question 12

give an example of a disease caused by co-dominant alleles

Answer 12

sickle cell anaemia

Question 13

define sex linkage

Answer 13

an inheritance pattern where the ratios are different in males and females

Question 14

many genetic diseases have been identified in humans but most are ———-

Answer 14

very rare

Question 15

what do radiation and mutagenic chemicals do?

Answer 15

they increase the mutation rate and can cause genetic disease and cancer

Question 16

define a mutation

Answer 16

a random change to the base sequence of a gene

Question 17

give two types of factors that will increase the mutation rate

Answer 17

• radiation (if it has enough energy to cause chemical changes in DNA) eg radioactive isotopes such as gamma rays and alpha particles, UV radiation and X rays
• chemical substances

Question 18

what is the origin of genetic diseases?

Answer 18

mutations in body cells are eliminated when the individual dies but mutations in cells that develop into gametes can be passed onto offspring.

Question 19

blood group A

Answer 19

IAIA or IAi

Question 20

blood group B

Answer 20

IBIB or IBi

Question 21

blood group AB

Answer 21

IAIB

Question 22

blood group O

Answer 22

ii

Question 23

describe how alleles of blood group will affect red blood cells

Answer 23

all 3 alleles cause the production of a glycoprotein in the membrane of RBCs:

IA:
- alters the glycoprotein by addition of acetyl-galactosamine.
- people with IA blood have anti-B antibodies in their plasma

IB:
- alters the glycoprotein by addition of galactose
- people with IB blood have anti-A antibodies in their plasma

IAIB:
- alters glycoprotein by addition of acetyl-galactosamine AND galactose
- neither anti-A nor anti-B antibodies will be produced by people with THIS genotype

ii:
- glycoprotein not altered
- both anti-A and anti-B antibodies in blood plasma

Question 24

what blood group is a universal donor?

Answer 24

O (because its glycoprotein is unaltered)

Question 25

what blood group is a universal recipient?

Answer 25

AB (because it does not have any antibodies in its plasma)

Question 26

what 2 things will happen if blood of the wrong group is given to someone?

Answer 26

1. agglutination, causing RBCs to clump together to facilitate phagocytosis
2. white blood cells will perforate RBC’s membrane, causing toxins/toxic substances to come out and causing the person to feel unwell

Question 27

describe cystic fibrosis

Answer 27

• due to recessive allele of the CFTR gene, located on chromosome 7
• gene product is a chloride ion channel that is involved in the secretion of sweat, mucus and digestive juices
• recessive alleles= channel does not function properly
• sweat produced contains excessive amounts of NaCl, but digestive juices and mucus w insufficient NaCl produced
• not enough water moves by osmosis into secretions making them viscous
• sticky mucus builds up in lungs causing infections
• pancreatic duct blocked so digestive enzymes do not reach small intestine

Question 28

describe Huntington’s disease

Answer 28

• due to dominant allele of the HTT gene, located on chromosome 4
• gene product is a protein called huntingtin (function unknown)
• allele results in degenerative changes in the brain
• symptoms start between 30 and 50, people have already started having kids by then
• causes changes in behaviour, thinking and emotions

Question 29

describe red-green colour blindness

Answer 29

• caused by a recessive allele of a gene for of the photoreceptor proteins, which are made by cone cells in the retina of the eye and detect specific wavelength ranges of visible light
• males; if their X chromosome carries the allele then it will be expressed in the phenotype
• females; both X chromosomes must carry the allele in order for it to be expressed

Question 30

describe haemophilia

Answer 30

• inability to make Factor VIII, one of the proteins involved in the clotting of blood
• recessive allele located on the X chromosome

Question 31

give an example of the consequences of nuclear bombing and accidents at nuclear power stations

Answer 31

• hiroshima and nagaasaki
• nuclear accidents at Three Mile island and Chernobyl