3.4 Inheritance

Question 1

Gregor Johann Mendel

Answer 1

• Discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed

Question 2

Law of Segregation

Mendel’s first law

Answer 2

When gametes form, alleles are separated so that each gamete carries only one allele for each gene.
- Gametes are haploid so contain only one allele of each gene.
- The two alleles of each gene separate into different haploid daughter nuclei during meiosis.

Question 3

Law of independent assortment

Mendel’s second law

Answer 3

Alleles of different genes assort independently of one another during gene formation.

Question 4

Allele

Answer 4

Different forms of the same gene

Question 5

Genotype

Answer 5

The two alleles an individual has for a gene

Question 6

Phenotype

Answer 6

The trait (observable characteristics) caused by the genotype

Question 7

Dominant

Answer 7

An allele which masks the effects of another (recessive) allele
- Shown with a capital letter

Question 8

Recessive

Answer 8

An allele which is masked by a dominant allele, in terms of the trait that is expressed.
- Shown with a lowercase letter

Question 9

Homozygous

Answer 9

2 identical alleles (e.g. tt)

Question 10

Heterozygous

Answer 10

Two different alleles (e.g. Tt)

Question 11

Zygotes

Answer 11

When a sperm and ovum fuse in fertilisation , they form a single diploid cell called a zygote
Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.

Question 12

Co-dominanance

Answer 12

Alleles that jointly affect the phenotype when present together
- Both are independently and equally expressed

Question 13

Incomplete dominance

Answer 13

Alleles that jointly affect the phenotype when present together
- The phenotype of the heterozygous is a blend of the dominant and recessive characteristics

Example: the pink flower resulting from the red and white flowers

Question 14

Monohybrid cross

learn punnet square

Answer 14

Step 1: two homozygous parents produce heterozygous offspring, this tells us the dominant gene
Step 2: the heterozygous offspring are crossed giving us a 3:1 ratio dominant:recessive

Question 15

Sex linked diseases

Answer 15

• Sex-linked traits are traits associated with the X chromosome because the X chromosome carries more genetic information.
• We are always assuming that the gene is carried on the X chromosome (NOT ON Y CHROMOSOME)
• Males only have one allele
• Females have two alleles, if the allele is recessive they can be carriers of sex linked diseases

Note: alleles carried on X chromosomes should be shown as superscript letters on an upper case X, such as Xh.

Question 16

Cystic fibrosis

Autosomal recessive disease

Answer 16

• Autosomal recessive disease
• Caused by a mutation in a gene on chromosome 5
• The person has to carry both recessive alleles to express the diseased phenotype
• People with CF produce abnormally thick and sticky mucus in their lungs and airways
• They are more likely to get respiratory infections
• Daily physiotherapy helps to relieve congestion, while antibiotics can fight infection
• The disease blocks tubes that take enzymes to the gut meaning food is not digested properly, leaving the person short of essential nutrients.

Question 17

Huntington’s disease

Autosomal dominant disorder

Answer 17

• Autosomal dominant disease
• The person only needs one dominant gene to express the diseased phenotype
• Caused by a mutation on a gene on chromosome 4
• Alters the structure of an important protein called the Huntington protein that supports brain activity
• Leads to neurodegeneration
• The symptoms usually develop in middle age, and include tremors, mood changes, memory loss and the inability to concentrate

Question 18

Sex linked inheritance examples

Answer 18

• Red green colour blindness
• Haemophilia: protein needed for blood clotting is not made

Both are sex-linked recessive disorders
- This means that females are less likely to express these disorders as they would need two recessive alleles while males only need one because the Y chromosome does not carry a the same gene that can cover up the problem with a dominant allele.

Question 19

Pedigrees tricks

Answer 19

Autosomal dominant:
- One of the parent has to have the allele
- Seen in every generation

Autosomal recessive:
- Skips generations
- A child has but none of the parents do

Sex linked
- Usually only males get it
- If a female gets it then her father must have it

Question 20

Radiation and causes of mutation

Answer 20

• Chemical mutagens
• Radiation

Increase the mutation rates and can cause genetic diseases and cancer

Question 21

Nuclear bombing of Hiroshima

Answer 21

• Over 100 000 people died around the time of the bombing from fires, the explosive shockwave, and Acute Radiation Syndrome (ARS).
• ARS occurs when a person is exposed to a large dose of ionising radiation.
• Lower doses cause nausea and headache, larger doses also cause seizures and fever, and very high doses cause death.
• Individuals who survived higher risk of certain types of cancer, especially leukemia, due to the increased number of mutations.

Question 22

Accident at Chernobyl

Answer 22

• Released far more radioactivity than the atomic bomb used in Hiroshima
• failed test led to the meltdown of one out of four reactors
• A complete meltdown was averted by workers who remained in highly radioactive conditions to stabilise the plant, some of whom died of ARS.

Question 23

Explain Mendel’s pea experiment

Answer 23

• First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
• Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
• These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)

Findings from results:
1. 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
2. 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 24

What happens to the two alleles of each gene during meiosis?

Answer 24

The two alleles of each gene separate into different haploid daughter nuclei during meiosis.

Question 25

The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes. Which genders are more prone to dominant and recessive traits?

Answer 25

As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous.
Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease).

Human males have only one X chromosome (and therefore only one allele). X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele.

• Females can be a carrier of a disorder without expressing it (heterozygote for a recessive disease condition)
• Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
• Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)

Question 26

Can radiation and mutagenic chemicals increase the mutation rate of cell DNA and cause genetic disorders and cancer?

Answer 26

Yes