Inheritance, Genes & Cell Division (Cambridge (CIE) IGCSE Biology)

True.

The father determines the biological sex of the child because he can pass on either an X or a Y chromosome.

The probability of a child inheriting two X chromosomes is 1/2 or 50 %.

The two stages of protein synthesis are:

• the DNA sequence is copied into mRNA (transcription)

• the mRNA works with a ribosome to assemble amino acids into a protein (translation)

The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.

The sequence of amino acids determine how the protein folds into its specific 3D shape. This is important for the functioning of the protein.

Protein synthesis is the process by which proteins are made in a cell. It is split into two parts:

• Transcription which copies the DNA code into mRNA

• Translation which uses the mRNA to produce a protein at a ribsome

The events of protein synthesis that take place within the nucleus are that mRNA copies a gene's DNA sequence.

False.

The DNA stays in the nucleus and the mRNA travels to the ribosome for translation.

mRNA (messenger RNA) is a copy of a gene.

It is a single-stranded RNA molecule that carries genetic information from DNA to ribosomes for protein synthesis.

The events of protein synthesis that occur within the cytoplasm (translation) are:

• the mRNA passes through ribosomes

• the ribosome assembles amino acids into protein molecules

• the specific sequence of amino acids is determined by the sequence of bases in the mRNA

Translation occurs in the cytoplasm of the cell, specifically at ribosomes.

False.

DNA is transcribed and translated one gene at a time. Each gene will produce a different protein.

False.

Most genes in a cell are not expressed ("switched off") to conserve energy and resources.

DNA controls cell function by controlling the production of proteins through gene expression.

Three types of proteins that can be produced through gene expression are enzymes, membrane carriers, and receptors for neurotransmitters.

A diploid nucleus contains two sets of chromosomes, totalling 46 chromosomes in humans.

A haploid nucleus contains one set of unpaired chromosomes, totalling 23 chromosomes in human gametes.

Humans have 23 different chromosomes in each cell. In a diploid cell each chromosome comes in a pair.

False.

Most body cells are diploid, but gametes (sex cells) are haploid.

A human diploid cell has a total of 46 chromosomes (23 pairs).

Mitosis is nuclear division that results in the production of genetically identical cells. It is used for growth, repair, and asexual reproduction.

False.

Mitosis results in genetically identical cells.

Before mitosis each chromosome replicates, producing two identical copies of each chromosome; one for each of the two daughter cells.

Mitosis is important in the body to produce new cells for growth, repair of damaged tissues, replacement of cells and asexual reproduction.

A mitotic division produces two genetically identical daughter cells. The daughter cells are genetically identical to each other and to the parent cell.

The gametes (i.e. sperm, egg cells, pollen and ovules) are not produced by mitosis, instead they are produced by meiosis.

Human cells produced by mitosis contain 23 pairs of, or 46, chromosomes.

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.

Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells. The cells produced by meiosis are gametes (sex cells).

Meiosis is essential for sexual reproduction as it produces gametes (sperm cells and egg cells) with half the normal chromosome number.

False.

Meiosis results in genetically different cells.

During meiosis the chromosome number is halved from diploid (46) to haploid (23). For this reason meiosis is known as reduction division.

Meiosis is crucial because it ensures that gametes (sperm and egg cells) have half the chromosome number, allowing for the correct chromosome number after fertilisation.

Homozygous: two identical alleles of a particular gene

Heterozygous: two different alleles of a particular gene

The genotype is the genetic make-up of an organism, including the types of alleles present.

The phenotype is the observable characteristics of an organism. This can be visibly observed (e.g. eye colour) or tested for (e.g. blood type).

A Punnett square is a diagram which shows the potential allele combinations that can result from a genetic cross between two individuals.

False.

Two recessive alleles are needed in the genotype in order for it to be expressed in the phenotype.

Dominant alleles are usually represented by a capital letter whereas recessive alleles are represented by a lower case letter.

If a tall pea plant with genotype TT is crossed with a short pea plant with genotype tt, all offspring would be heterozygous with the genotype Tt. This is because a T allele is inherited from the tall parent and a t allele is inherited from the short parent.

If a tall pea plant with genotype Tt is crossed with another tall pea plant with genotype Tt, the phenotype ratio of the offspring would be 3: 1 (3 tall and 1 short). This is because the cross would result in 1 x TT (tall), 2 x Tt (tall) and 1 x tt (short).

When a homozygous dominant individual is crossed with a homozygous recessive individual there is 100 % probability that the offspring will display the dominant phenotype. This is because all offspring will be heterozygous.

If two heterozygous individuals are crossed there is a 1/4, 0.25 or 25 % probability of offspring having a homozygous recessive genotype.

The predicted ratio of offspring characteristics from two heterozygous parents is 3:1 dominant vs recessive characteristic.

Family tree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family.

Codominance occurs when both alleles of a genotype in heterozygous organisms are expressed in the phenotype of an individual.

False.

Codominance results in both alleles being expressed equally in the phenotype.

The alleles for blood group are I^A, I^B and I^o.

The alleles for blood group show the following relationship:

• I^A and I^B are codominant

• I^A and I^B are dominant to I^O

True.

A person with blood type AB has the genotype I^AI^B.

A sex-linked characteristic is one where the alleles controlling the trait are found on the sex chromosomes, typically the X chromosome.

Sex linkage makes the characteristic more common in one sex than in the other.

Males are more likely to show sex-linked recessive conditions because they only have one X chromosome, so a recessive allele on that chromosome will always be expressed.

A carrier is a female with one recessive allele for a sex-linked trait that is masked by a dominant allele. She doesn't have the condition but can pass it on to offspring.

The probability that a son born to a carrier female will have colour blindness is 50 %:

For any son born from the parents shown, the probability that he will have colour blindness is 50 %.

Note that the vision of the father is irrelevant here, as any son will only inherit the Y chromosome from his father.

False.

The Y chromosome is much smaller and carries fewer genes than the X chromosome.

In a cross between a male with colour vision and a female carrying the allele for colour blindness, there is a 25 % chance of producing a child who is a carrier.