Topic 3 Genetics

Question 1

Gene

Answer 1

A heritable factor that consists of a length of DNA and influences a specific trait.

Question 2

Loci

Answer 2

A gene occupies a specific position on a chromosome called the loci.

Question 3

Allele

Answer 3

A specific form of a gene. E.g. (blue, green, or brown eyes). Alleles only differ from another by one or a few bases.

Question 4

Gene mutation

Answer 4

A change in the nucleotide sequence of a section of DNA coding for a specific trait. New alleles are formed by mutation. Gene mutations can be beneficial, detrimental or neutral.

Question 5

Sickle Cell Anaemia

Answer 5

An example of a disorder caused by a gene mutation. The disease allele arose from a base substitution mutation – where a single base was changed in the gene sequence. Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin. The amino acid change (Glu → Val) alters the structure of haemoglobin, causing it to form insoluble fibrous strands. The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired.
The formation of fibrous haemoglobin strands changes the shape of the red blood cell to a sickle shape. The sickle cells may form clots within the capillaries, blocking blood supply to vital organs and causing myriad health issues. The sickle cells are also destroyed more rapidly than normal cells, leading to a low red blood cell count (anaemia).

Question 6

Genome

Answer 6

The totality of genetic information of a cell, organism or organelle. Includes all genes as well as non-coding DNA sequences. The human genome consists of 46 chromosomes (barring aneuploidy). The entire base sequence of human genes was sequenced in the Human Genome Project.

Question 7

Gene comparisons

Answer 7

The number of genes present in an organism will differ between species and is not a valid indicator of biological complexity. The number of genes in rice (Oryza sativa) is estimated as being between 32,000 – 50,000
The number of genes in humans is estimated as being between 19,000 – 25,000.

Question 8

Prokaryotic genetics

Answer 8

Prokaryotes do not possess a nucleus – instead genetic material is found free in the cytoplasm in a region called the nucleoid. The genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore). The DNA of prokaryotic cells is naked – meaning it is not associated with proteins for additional packaging. In addition to the genophore, prokaryotic cells may possess additional circular DNA molecules called plasmids.

Question 9

Eukaryotic genetics

Answer 9

The genetic material of eukaryotic cells consist of multiple linear molecules of DNA that are associated with histone proteins. The packaging of DNA with histone proteins results in a greatly compacted structure, allowing for more efficient storage.

Question 10

Homologous pairs

Answer 10

Sexually reproducing organisms inherit their genetic sequences from both parents .This means that these organisms will possess two copies of each chromosome (one of maternal origin ; one of paternal origin). These maternal and paternal chromosome pairs are called homologous chromosomes. Homologous chromosomes are chromosomes that share:
- The same structural features (e.g. same size, same banding patterns, same centromere positions)
- The same genes at the same loci positions (while the genes are the same, alleles may be different)
Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation

Question 11

Diploid versus Haploid

Answer 11

As sexually reproducing organisms receive genetic material from both parents, they have two sets of chromosomes (diploid). To reproduce in turn, these organisms must create sex cells (gametes) with half the number of chromosomes (haploid). When two haploid gametes fuse, the resulting diploid cell (zygote) can grow and develop into a new organism.

Diploid
Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 2n):
- These nuclei will possess two gene copies (alleles) for each trait
- All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
- Diploid cells are present in most animals and many plants

Haploid
Nuclei possessing only one set of chromosomes are haploid (symbolised by n)
- These nuclei will possess a single gene copy (allele) for each trait
- All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
- Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)

Question 12

Autosomes verus Heterosomes

Answer 12

In humans, sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes).
Females possess two copies of a large X chromosome (XX) and males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY).

The Y chromosome contains the genes for developing male sex characteristics (specifically the SRY gene). In its absence of a Y chromosome, female sex organs will develop. The sex chromosomes are homologous in females (XX) but are not homologous in males (XY)

The father is always responsible for determining the sex of offspring.

The remaining chromosomes in the organism are called autosomes (they do not determine sex)

Question 13

Karyogram

Answer 13

Karyotypes are the number and types of chromosomes in a eukaryotic cell. The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram.

Question 14

Cairns’ technique for measuring the length of DNA molecules

Answer 14

Autroradiography.

Question 15

Meitotic Division

Answer 15

Meiosis is the process by which sex cells (gametes) are made in the reproductive organs. It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei. The process of meiosis consists of two cellular divisions:

• The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)
• The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)

Question 16

Stages of Meitosis

Answer 16

Meiosis I: P(I) - M (I) - A(I) - T(I)

Meiosis II: P(II) - M (II) - A(II) - T(II)

Question 17

Genetic variation

Answer 17

The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring. The three main sources of genetic variation arising from sexual reproduction are:

• Crossing over (in prophase I)
• Random assortment of chromosomes (in metaphase I)
• Random fusion of gametes from different parents

Question 18

Non-disjunction

Answer 18

The chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy).
The failure of chromosomes to separate may occur via:
- Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
- Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)

E.g. Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21).

Studies showing age of parents influences chances of non-disjunction

Question 19

Karyotyping

Answer 19

The process by which chromosomes are organised and visualised for inspection. Karyotyping is typically used to determine the gender of an unborn child and test for chromosomal abnormalities.

Chorionic villi sampling involves removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix. It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)

Amniocentesis involves the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle. It is usually conducted later than CVS (~16 weeks of pregnancy) with a slightly lower risk of miscarriage (~0.5%).

Question 20

Mendel’s Laws

Answer 20

Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene
Principle of Dominance: Recessive alleles will be masked by dominant alleles (Not all genes show a complete dominance hierarchy – some genes show co-dominance or incomplete dominance)

Question 21

Haploid gametes

Answer 21

Gametes are haploid sex cells formed by the process of meiosis – males produce sperm and females produce ova. During meiosis I, homologous chromosomes are separated into different nuclei prior to cell division
As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs. Consequently, as gametes contain only one copy of each chromosome they therefore carry only one allele of each gene