Genetics 3.4 Inheritance

Question 1

What is a phenotype?

Answer 1

Phenotype is an observable trait or characteristic.

Question 2

What is a genotype?

Answer 2

Genotype is an organism’s genetic make up

Question 3

What is an allele?

Answer 3

An allele is a version of a gene.

Question 4

What is a dominant allele?

Answer 4

An allele which determines the phenotype of a heterozygote.

Question 5

What is a recessive allele?

Answer 5

An allele which can only influence the phenotype of a homozygote.

Question 6

What is the locus?

Answer 6

A position in a chromosome.

Question 7

What is homozygous?

Answer 7

An organism is homozygous when it has two identical alleles of the same gene.

Question 8

What is heterozygous?

Answer 8

An organism is heterozygous when it has two different alleles of the same gene.

Question 9

When a test cross is performed what happens?

Answer 9

Testing a suspected heterozygote by crossing it with a known homozygous recessive.

Question 10

What is a monohybrid cross?

Answer 10

A monohybrid cross determines the allele combinations of offspring for one particular gene only (HL students may refer to topic 10.2 for dihybrid crosses)

Question 11

A parent organism of unknown genotype is mated in a test cross. Half of the offspring have the same phenotype as the parent. What can be concluded from this result?

Answer 11

The parent of unknown genotype is heterozygous.

Question 12

The gene for brown hair is B and the gene for blond hair is b

a) Write down the genotype of a woman who is heterozygous for brown hair.
b) Write down the genotype of a man who has blond hair.
c) Work out the possible offspring of these two people.

Answer 12

a) Bb
b) bb
c) 50% Brown hair & 50% blond hair

Question 13

What is the use of test cross?

Answer 13

A genetic cross is a means of determining the genetic characteristics of potential offspring based on the genetic characteristics of the prospective parents. By test crossing we can determine the parent’s unknown genotype.

Question 14

What is codominance?

Answer 14

Codominance means that two different alleles both affect the phenotype of a heterozygote.

Question 15

How many alleles can a gene have?

Answer 15

A gene can have multiple alleles that means more than 2 versions of the same gene.

Question 16

What is an example of codominance and multiple alleles?

Answer 16

ABO blood groups have codominant alleles as well as more than 2 alleles for the same gene. (Recessive allele is i which does not produce glycoprotein. Phenotype = Blood group O)

Question 17

What is sex linkage?

Answer 17

Sex linkage occurs when a gene is located on one the sex chromosomes (chromosome #23)

Question 18

What are examples of sex linkage?

Answer 18

Red Green Color Blindness & Hemophilia are both examples of X-linked recessive conditions The gene loci for these conditions are found on the non-homologous region of the X chromosome (they are not present of the Y chromosome) As males only have one allele for this gene they cannot be a carrier for the condition This means they have a higher frequency of being recessive and expressing the trait Males will always inherit an X-linked recessive condition from their mother Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents

Question 19

Can males be both homozygous and heterozygous with respect to sex-linked genes?

Answer 19

No. As human females have two X chromosomes (and therefore two alleles for any given X-linked gene), they can be either homozygous or heterozygous Males only have one X chromosome (and therefore only one allele) and are hemizygous

Question 20

Explain that female carriers are heterozygous for X-linked recessive alleles

Answer 20

An individual with a recessive allele for a disease condition that is masked by a normal dominant allele is said to be a carrier Carriers are heterozygous and can potentially pass the trait on to the next generation, but do not suffer from the defective condition themselves Females can be carriers for X-linked recessive conditions because they have two X chromosomes - males (XY) cannot be carriers Because a male only inherits an X chromosome from his mother, his chances of inheriting the disease condition from a carrier mother is greater

Question 21

What is the cause of most genetic diseases?

Answer 21

Many genetic diseases in humans are caused due the recessive allele of autosomal genes, although some genetic diseases are due to dominant or codominant alleles.

Question 22

What is cystic fibrosis?

Answer 22

• Autosomal disease • Recessive
• C = healthy/ c = cystic fibrosis
• Suffers produce a thick mucus in their lungs which gives them trouble breathing
• No cure, no effective treatment.

Question 23

What is Huntington’s disease?

Answer 23

• Autosomal Dominant
• H = Huntington’s disease h = healthy
• patients incur damage to nerve tissues, mental decline aged 30+

Question 24

Explain why large sample sizes are necessary in genetics investigations

Answer 24

• Large sample sizes increases reliability of the data
• Large sample sizes reduce the impact of random effects/ small sample sizes increases the impact of random effects

Question 25

What is mutagen?

Answer 25

Mutagens a physical or chemical factor that causes DNA mutations • Physical (UV rays, gamma radiation, X-rays) • Chemical (polonium, radium, dioxins, carbon monoxide, uranium)

Question 26

What are the consequences of radiation after a nuclear bomb?

Answer 26

• Higher rate of cancer among survivors • Blood disorders (e.g. Leukemia) • Down syndrome • Soil contamination