3.4 Inheritance

Question 1

gametes

Answer 1

haploid sex cells formed by the process of meiosis – males produce sperm and females produce eggs

Question 2

homozygous

Answer 2

the maternal and paternal alleles are the same

Question 3

heterozygous

Answer 3

the maternal and paternal alleles are different

Question 4

genotype

Answer 4

The gene composition (i.e. allele combination) for a specific trait

Question 5

phenotype

Answer 5

The observable characteristics of a specific trait

Question 6

dominance

Answer 6

The dominant allele will mask the recessive allele when in a heterozygous state

Question 7

co-dominance

Answer 7

occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual

Question 8

An autosomal recessive genetic disease

Answer 8

will only occur if both alleles are faulty

Question 9

How did Mendel discover the Principles of Inheritance?

Answer 9

1. Mendel crossed varieties of pea together by transferring the male pollen from one variety to the female parts in flowers of another variety.
2. He collected the pea seeds that were formed as a result and grew them to find out what their characters were.
3. Mendel repeated each cross with many pea plants; he did this experiment with seven different pairs of characters and his results reliably demonstrated the principles of inheritance in peas
4. Mendel’s theory explains the basis of inheritance in all plants and animals.

Question 10

How are ABO blood groups an example of co-dominance? What are the genotypes of blood groups A, B, O, AB?

Answer 10

The genotypes IAIA and IAi give blood group A
The genotypes IBIB and IBi give blood group B
i is recessive to both IA and IB
The genotype ii gives blood group O
Neither IA and IB is dominant over the other allele
The genotype IAIB gives blood group AB

Question 11

How are genetic diseases caused by recessive alleles?

Answer 11

A person has one allele for the genetic disease and one dominant allele → symptoms of the disease are not shown, but they can pass on the recessive allele to their offspring called carriers (eg. Cystic fibrosis)
Both parents should be carriers or homozygous recessive for the offspring to have the disease

Question 12

What are the other causes of genetic diseases?

Answer 12

Some genetic diseases are caused by a dominant allele; impossible to be a carrier (eg. Huntington’s disease)
A very small proportion of genetic diseases are caused by co-dominant alleles
Sickle-cell anemia: individuals that have one Hb and one Hb allele do not have the same characteristics as those have two copies of either allele

Question 13

Explain the causes of cystic fibrosis.

Answer 13

Due to recessive allele → autosomal recessive
Usually the parent are both carriers of the recessive allele for the disease
Carrier does not affect the phenotype because a dominant allele is also present
The Punnett grid shows that the probability of cystic fibrosis in a child of two carrier parents is 25%
The recessive alleles result in digestive juices and mucus being secreted with insufficient sodium chloride
Not enough water moves by osmosis into the secretions, making them very viscous
Sticky mucus builds up in the lungs causing infections and the pancreatic duct is blocked so digestive enzymes secreted by the pancreas do not reach the small intestine.

Question 14

Explain the causes of Huntington’s disease.

Answer 14

Due to dominant allele → autosomal dominant
A person can develop the disease if only one of their parents has the allele because it is dominant
Changes to behaviour, thinking, and emotions become severe
Life expectancy after the start of symptoms is about 20 years
A person with the disease needs full nursing care and usually succumbs to heart failure, pneumonia or some other infectious disease

Question 15

What is sex-linkage? How does it cause some diseases?

Answer 15

Sex-linkage: association of a characteristic with the sex of the individual, because the gene controlling the characteristic is located only on a sex chromosome
X chromosome is relatively large than Y chromosome and has important genes on it; sex-linkage is almost always due to genes on the X chromosome
Pattern of inheritance of the genes differ in males and females because females have two X chromosomes (two copies of each gene) and males have one X chromosome
Only females can be carriers of recessive alleles of sex-linked genes and conditions due to these alleles are much more frequent in males than in males

Question 16

Explain how red-green colour blindness is caused.

Answer 16

Caused by a recessive allele of a gene for one of the photoreceptor proteins
The proteins are made by cone cells in the retina of the eye and detect specific wavelength ranges of visible light
If the X chromosome from mother carries red-green colour-blindness allele then the son will be red-green colour-blind, as males have only one X chromosome which they inherit from their mother

Question 17

Explain how hemophilia is caused.

Answer 17

Caused by a recessive allele of a gene
Due to inability to make Factor VIII, one of the proteins involved in the blood clotting
Life expectancy is only about ten years if hemophilia is untreated
Females can be the carriers of the recessive hemophilia allele but the disease is only developed if both of their X chromosomes carry the allele

Question 18

What are the two factors that can increase the mutation rate?

Answer 18

Radiation: increases the mutation rate if it has enough energy to cause chemical changes in DNA; gamma rays and alpha particles from radioactive isotopes, short-wave ultraviolet radiation and X-rays are all mutagenic
Mutagenic chemicals: cause chemical changes in DNA (eg. benzopyrene and nitrosamines found in tobacco smoke and mustard gas used as a chemical weapon in the WW1)

Question 19

What are the consequences of Nuclear Bombing and Accidents in Hiroshima and Nagasaki?

Answer 19

Effect: mutations leading to stillbirths, malformation, death