Genes and Inheritance

Chromosomes :

Structure :

·                Chromosomes are long, thread – like structures, consisting of DNA, with a protein backbone.

·                They are joined by circular centromeres.

·                Chromosomes consist of chain of genes, that are made up of DNA.

·                Each DNA molecule is a double helix made up of two strands of deoxyribose sugars, phosphates and nitrogenous bases : Adenine – Thymine or Cytosine – Guanine.

Function :

·                Genes determine phenotype.

·                The DNA sequence on genes codes for a particular protein or polypeptide.

·                For example, structural proteins, like collagen, or functional proteins, like enzymes.

·                Copies of the DNA sequence are made during mitosis.

·                The genes are passed to the next generation to determine their characteristics.

Mutation :

·                Mutation is a random, sudden and spontaneous change in the structure of the genes or the DNA or a change in the number of chromosomes in the nucleus of a cell.

·                It can lead to the cell coding for the wrong protein that has a different function.

·                Hence, the proteins and enzymes produced by the cell changes.

·                Mutation can change the structure of the cell or change the reactions in the cell.

·                Mutation can change the phenotype.

·                Most cells are ideal for their purpose.

·                Hence, such changes can lead to the cell being less efficient.

·                Mutation can cause uncontrolled cell division, leading to cancers.

·                Mutation can cause sickle cell anaemia.

Mutagens are

·                                  X rays

·                                  UV light

·                                  Ionising radiation.

Haemophilia occurs more in males :

·                The sex chromosomes of males are XY and sex chromosomes of females are XX.

·                The recessive allele for haemophilia is carried on the X chromosome (23^rd chromosome).

·                The allele for normal blood clotting is dominant.

·                Heterozygous females are carriers and they do not show the condition.

·                The recessive allele for haemophilia must be present on both X chromosomes to cause it in females / The female must be homozygous recessive.

·                If the recessive allele is present on a single X chromosome in males, it causes condition.

·                The allele for haemophilia is recessive and is rare in the population.

·                Haemophilic females are unlikely to reach breeding age.

·                Blood loss during menstruation can cause early death in females.

Sex linked diseases :

·                These are caused a gene, carrying the recessive allele.

·                The gene is carried on the X chromosome / the 23^rd chromosome

·                Males have XY

·                A single allele needs to be inherited to show the condition

·                Females have XX

·                The female may be heterozygous for the allele and be a carrier – not show the condition

Note : A disease is not sex – linked it occurs in both genders.

Red – green colour blindness :

·                The allele is rare in the population

·                A colour blind male carries the recessive allele on the X chromosome only

·                Y is passed to the son

·                X is passed to the daughter, but the other X is likely to carry the dominant allele

·                To show the condition, the female must inherit it from both parents

Note : X has sector unpaired on Y, so the feature on it is always shown in males.

Sickle cell anaemia :

·                A person heterozygous for sickle – cell anaemia, performing vigorous exercise is likely to develop sickle shaped red blood cells as more oxygen is used up in aerobic respiration, causing concentration of oxygen in blood to fall.

·                The symptoms of sickle cell anaemia are more likely to be reduced by suitable blood transfusions. New red blood cells are present in blood. The new red blood cells are normal, so proportion of sickle shaped cells are reduced. Symptoms are reduced. Normal red blood cells are introduced. These pick up oxygen in blood, causing oxygen level in blood to rise.

·                A severe attack of the condition if untreated may result in permanent damage to body organs or even death, as sickle shaped cells will block blood vessels. If this happens in the heart, it leads to heart attack. If this happens in the brain, it leads to organ / brain damage.

Note :  Body size is difficult to study using a family pedigree as it is affected by many factors, such as,

·                food

·                disease

·                exercise

·                environmental factors

If a disorder is caused by a recessive allele

·                Non sufferers produce a sufferer child

·                The condition appears in homozygotes only

·                Sufferers are few in number

If the condition is caused by a dominant allele

·                Both / One parent would suffer if heterozygous

·                Sufferers are more in number