17 Inheritance

Question 1

DNA

Answer 1

A large molecule made from bases, and which carries genetic information in the form of genes.

Deoxyribonucleic acid

Question 2

Chromosome

Answer 2

The DNA in the nucleus of a cell is coiled and packed to form a thread-like structure called a chromosome

Question 3

Genes

Answer 3

A length of DNA that codes for a protein.

Question 4

Allele

Answer 4

An alternative version of a gene. A section of DNA that codes for a particular protein.

Question 5

Haploid and diploid

Answer 5

a gamete has a haploid nucleus – it contains a single set of unpaired chromosomes

a body cell has a diploid nucleus – it contains two sets of chromosomes.

Question 6

The DNA of human body cells contains two sex chromosomes:

Answer 6

males have one X chromosome and one Y chromosome, XY
females have two X chromosomes, XX.

a sperm cell (the male gamete) has one X chromosome or one Y chromosome
an egg cell (the female gamete) contains one X chromosome.

Question 7

DNA consists of

Answer 7

DNA consists of two strands that coil together to form a double helix (two spirals that wrap around each other). Each strand contains chemicals called bases.

There are four bases, A, T, C and G. They always pair up this way:
A with T
C with G.

These are called complementary base pairs

Question 8

Why sequence matters

Answer 8

The sequence (order) of bases differs from gene to gene. The sequence of bases in a gene is its genetic code: it determines the order in which amino acids are joined together to make a specific protein

Question 9

Gene expression

Answer 9

Different sequences produce different proteins. These proteins can include:
enzymes
antibodies
receptors for neurotransmitters

Every cell in the body has a copy of every gene, but different genes are expressed at different stages in the life of the cell, and some genes may not be expressed at all.

Question 10

Protein synthesis

Answer 10

A DNA molecule in the nucleus unzips, exposing the bases in a gene.

An mRNA molecule is formed using the exposed bases of the DNA strand as a template. The bases in the mRNA molecule pair with the complementary bases in the DNA, so the mRNA molecule carries a copy of the gene.

The mRNA molecule moves out of the nucleus into the cytoplasm.

A ribosome reads the mRNA code and joins amino acids together to form a protein, following the specific sequence of bases in the mRNA.

Question 11

Mitosis

Answer 11

Mitosis is a type of nuclear division that gives rise to genetically identical cells. The cells produced by nuclear division are called daughter cells (even if they are not part of a female).

Question 12

The role of mitosis

Answer 12

When a zygote is formed during the fertilisation of an egg cell by a sperm cell, it consists of just one cell. The zygote will grow and develop as more cells are produced through mitosis.

After an injury, new cells are needed to repair the damaged tissues. Mitosis is responsible for the repair of damaged tissues and the replacement of cells.
It is also involved in asexual reproduction.

Question 13

Mitosis and cell division

Answer 13

In preparation for mitosis, each chromosome is replicated exactly so that there are two copies of each chromosome when the cell is about to divide.

This is when they become visible under the microscope as X-shaped structures because the two copies are joined together near the middle.

When the nucleus divides into two, one copy of each chromosome goes into each new cell during cell division.

This keeps the number of chromosomes in the two daughter cells the same as in the original cell.

Question 14

Stem cells

Answer 14

unspecialised cells that divide by mitosis, producing daughter cells that can become specialised for specific functions.

Stem cells are found at sites such as embryos and umbilical cords, and adult bone marrow (the spongy part inside bones).

These cells can differentiate so they become specialised for a particular function.

This allows them to form various tissues, organs and organ systems.

These cells are still genetically identical in an individual person, but their genes are expressed differently

Question 15

Miosis

Answer 15

Meiosis is a type of nuclear division that gives rise to cells that are genetically different

Meiosis is involved in the production of gametes. In humans:
male gametes (sperm cells) are produced in the testes
female gametes (egg cells) are found in the ovaries and mature there.

Meiosis occurs in the production of gametes of all organisms that reproduce sexually, including flowering plants.

Question 16

Meiosis and variation

Answer 16

Like mitosis, each chromosome is exactly duplicated before meiosis begins

In meiosis, the number of chromosomes is halved, so it is called a reduction division. Each cell will have one chromosome of each pair.

In meiosis, a diploid cell (with a nucleus containing two sets of chromosomes ) becomes four haploid cells (each with a nucleus containing a single set of unpaired chromosomes).

The haploid cells formed are genetically different from each other, and from the original diploid cell. This variation happens in two ways.

A process called crossing over occurs in an early stage of meiosis. The chromosomes in a pair of chromosomes swap parts with each other Two new chromosomes form, each of which is different from the two original chromosomes.

In later stages of meiosis, the cell divides twice to form four daughter cells

Question 17

Inheritance

Answer 17

Is the transmission of genetic information from generation to generation. An organism that receives genetic information from its parent (if asexual reproduction has occurred) or its parents (if sexual reproduction has occurred) is said to have inherited this information.

Question 18

Homozygous and heterozygous

Answer 18

homozygous if there are two identical alleles
heterozygous if there are two different alleles.

Question 19

Phenotype and genotype

Answer 19

The observable features of an organism.

An organism’s genotype is all the alleles present in that organism.

Question 20

Monohybrid cross

Answer 20

Breeding two individuals that differ in their alleles for a particular gene.

Question 21

dominant and recessive genes

Answer 21

A dominant allele is always expressed if it is present
A recessive allele is only expressed if the dominant allele is not present.

In practice, this means that a dominant allele is expressed if one or two copies are present, but a recessive allele is only expressed if two copies are present.

Question 22

genetic symbol

Answer 22

Use the same letter
Y(capital) represents the dominant allele
y(lowercase) represents the recessive allele.

Question 23

Codominace

Answer 23

A situation in which two different alleles for a gene influence the phenotype to the same extent, for example in human ABO blood groups.

Question 24

Blood group

Answer 24

ABO blood groups
People can belong to blood group A, B, AB or O.
Genotype Blood group
I A I A or I A I O: A
I B I B or I B I O: B
I A I B: AB
I O I O: O

Alleles I A and I B are both dominant to allele I O

Alleles I A and I B are codominant to each other – a person inheriting both alleles is blood group AB (not A or B).

Question 25

sex-linked characteristics

Answer 25

A characteristic that is determined by a gene carried on the sex chromosomes, usually the X chromosome.

Question 26

Disorders carried by by sex linkage

Answer 26

They include haemophilia (a blood-clotting disorder) and red-green colour blindness . The alleles for these characteristics are carried on the X chromosome:

The Y chromosome is shorter and therefore carries some genes that are not present on the X chromosome
The X chromosome carries many more genes than the Y chromosome because it is much longer. Many genes found on the X chromosome are not found on the Y chromosome

Question 27

Colour blindness

Answer 27

Females must inherit two copies of the X r allele to have colour blindness
Females can be carriers of the allele for colour blindness (have one X r but not have the disorder)
Males only need to inherit one copy of X r allele to have colour blindness.