Inheritance

Question 1

What did Gregor Mendel study?

Answer 1

The inheritance of characteristics in the garden pea.

Question 2

Why did Mendel choose the pea plant to study?

Answer 2

As they are easy to grow and have easily distinguishable characteristic.

Question 3

What is a phenotype?

Answer 3

An observable characteristic.

Question 4

What is a genotype?

Answer 4

A pair of alleles which determine a phenotype.

Question 5

What is the monohybrid ratio?

Answer 5

3 dominant phenotypes : 1 recessive phenotype

Question 6

What is Mendel’s first law of segregation?

Answer 6

The characteristics of an organism are determined by genes which occur in pairs. Only one member of a pair of genes can be represented in a single gamete.

Question 7

What is the dihybrid ratio?

Answer 7

9 showing two dominant phenotypes : 3 showing one dominant and one recessive phenotype : 3 showing one dominant and one recessive phenotype : 1 showing two recessive phenotypes

Question 8

What is Mendel’s second law of independent assortment?

Answer 8

Each pair of contrasted characters may be combined with either of another pair.

Question 9

When are Mendel’s laws able to be applied?

Answer 9

When the genes are on different chromosomes and not linked.

Question 10

What is a test cross used for?

Answer 10

It is used to determine the genotype of the organisms with the dominant phenotype.

Question 11

What is the monohybrid test cross?

Answer 11

The organism whose genotype is unknown is crossed with a homozygous recessive organism.
If the unknown genotype is homozygous dominant then all the offspring will show the dominant phenotype.
If the unknown genotype is hetrozygous then the ratio of dominant to recessive phenotypes will be 1:1.

Question 12

What is the monohybrid test cross ratio?

Answer 12

1 dominant phenotype : 1 recessive phenotype

Question 13

What is the dihybrid test cross?

Answer 13

It is used to determine the genotype of an organism showing two dominant phenotypes by crossing it with a homozygous recessive organism.
If the organism is homozygous then only one type of phenotype will be shown in the offspring.
if the organism is heterozygous then the ratio of phenotypes in the offspring will be 9:3:3:1.

Question 14

What is the dihybrid test cross ratio?

Answer 14

9 showing two dominant phenotypes : 3 showing one dominant and one recessive phenotype : 3 showing one dominant and one recessive phenotype : 1 showing two recessive phenotypes

Question 15

What is codominance?

Answer 15

Inheritance patterns when both alleles in a hetrozygous organism are equally expressed.

Question 16

What are examples of codominance?

Answer 16

-Coat colour in cattle
-Human blood groupings

Question 17

How do genes form recombinants?

Answer 17

During meiosis when homologous pairs of chromosomes come together during prophase I, crossing over may occur at chiasmata. If crossing over occurs between two genes, this separates alleles which were previously linked and allows them to combine in new recombinant genotypes. This is important as a source of variation.

Question 18

When can the Chi-squared test be used?

Answer 18

It is used to test the statistical significance of discrete variables. It can be used to determine if the results of a genetic cross are significantly different to expected results or whether the differences are due to chance alone.

Question 19

What does the null hypothesis always state in the Chi-squared test?∑

Answer 19

There is no significant difference between expected and observed results, and any deviation is due to chance alone.

Question 20

How can degrees of freedom be calculated?

Answer 20

(Number of categories) -1

Question 21

What is always the probability level?

Answer 21

5% or p=0.05

Question 22

What is the Chi-squared formula?

Answer 22

∑ (observed value-expected value)^2 / expected value

Question 23

How do you carry out a Chi-squared test?

Answer 23

1. Formulate a null hypothesis
2. Calculate the expected numbers from Mendelian ratios
3. Calculate the degrees of freedom
4. Choose a suitable probability level
5. Calculate the chi-squared value
6. Calculate the critical value
7. Formulate a conclusion

Question 24

What conclusion do you draw if the calculated value for Chi-squared is lower than or equal to the critical value?

Answer 24

The null hypothesis is accepted, and any deviation between observed and expected results is due to chance alone.

Question 25

What conclusion do you draw if the calculated value for Chi-squared is higher than the critical value?

Answer 25

The null hypothesis is rejected, and any deviation between observed and expected results is due to some other factor than chance alone.

Question 26

In what three ways does meiosis result in variety?

Answer 26

-Mixing the genotype of one parent with that of another increases variety of offspring. The haploid gametes fusing in fertilisation brings chromosomes from two different sources resulting in a unique new combination of genetic information. -The random distribution and independent assortment of homologous chromosomes on the equator of the spindle during metaphase I of meiosis. When the homologous chromosomes separate, the daughter cells contain different combinations of genetic information. -Crossing over during chiasmata formation during prophase I of meiosis. Equivalent parts of homologous chromosomes may be exchanged thus producing new genetic combinations and the separation of linked genes.

Question 27

How is a characteristic sex-linked?

Answer 27

If the gene which determines it are found on sex chromosomes.

Question 28

Who do fathers pass on x-linked traits to?

Answer 28

Their daughters.

Question 29

Who do mothers pass on x-linked traits to?

Answer 29

Their sons and daughters.

Question 30

What is haemophilia?

Answer 30

The inability of the blood to clot since blood-clotting factor 8 cannot be produced leading to slow and persistent bleeding, especially in the joints. The recessive allele is very rare and occurs on the X chromosome.

Question 31

What is Duchenne muscular dystrophy?

Answer 31

It is the progressive weakening of the muscles, caused by a sex-linked recessive allele of a gene that codes for a protein dystrophin, which stabilises the cell membranes of muscle fibres.

Question 32

Describe the features of sex-linked recessive alleles

Answer 32

-The are carried in the X chromosome -They are only expressed in females if both X chromosomes carry the allele -They are always expressed in the male, since the Y chromosome does not have a homologous locus for the gene

Question 33

What is a mutation?

Answer 33

A change in the amount or arrangement of the genetic material (DNA or RNA) in a cell.

Question 34

Describe the characteristics of mutations

Answer 34

-They are spontaneous, random events and mutation rates are normally very low, but in organisms with shorter life cycles and more frequent cell division, the rate of mutation is higher. -They are a source of genetic variation which results in evolution through nature. -Most mutations occur during crossing over in prophase I of meiosis or as a result of non-disjunction during anaphase I or II of meiosis.

Question 35

What are the two types of mutation?

Answer 35

Gene mutations, affecting a base in a single gene, or chromosomal mutations, affecting many genes.

Question 36

How can the rate of mutation be increased?

Answer 36

By mutagens, such as: -Ionising radiation, such as gamma, UV and x-ray -Certain chemicals, such as polycyclic hydrocarbons in cigarette smoke

Question 37

What is the name for a mutagen that causes cancer?

Answer 37

A carcinogen.

Question 38

What happens when a proto-oncogene undergoes a mutation?

Answer 38

They form oncogenes, which are involved in causing uncontrolled cell division to form a cancer.

Question 39

How is sickle-cell anemia caused by a mutation?

Answer 39

It is caused by a single mutation in the gene coding for haemoglobin, creating haemoglobin S, which makes the blood cells sickle shaped and is less efficient at carrying oxygen, so the person suffers from anemia.

Question 40

What are the two types of chromosome mutations?

Answer 40

-Polyploidy, additional complete sets of chromosomes. -Additional chromosomes, during anaphase I of meiosis a pair of homologous chromosomes fail to separate. This is non-disjunction and is often lethal.

Question 41

What is a frequent consequence of non-disjunction in humans?

Answer 41

Down's syndrome, where the 21st homologous chromosomes fail to separate and a gamete can contain 24 chromosomes. The fusion of the gamete with a normal one results in the zygote having 47 chromosomes.

Question 42

What is epigenetics?

Answer 42

The control of gene expression by factors other than changes in DNA sequencing.

Question 43

What is DNA methylation?

Answer 43

The addition of methyl groups to bases, which prevents those bases being recognised and reduces the ability of that gene to be expressed.

Question 44

What is gene expression?

Answer 44

The process by which information from a gene is used in the synthesis of a functional product.

Question 45

How can histone proteins be modified?

Answer 45

If the histone is coiled more tightly, it can prevent gene expression. If the histone is coiled more loosely, it can increase gene expression.

Question 46

What can different epigenetic modifications occurring in cells of different tissues result in?

Answer 46

Different expressions of the same gene in different parts of the same organism.