Topic 3 : Genetics : 3.4 Inheritance - IBDP, MYP, AP, iGCSE, A-Level

Mendel’s pea plants:

Mendel was known as the father of genetics
Mendel performed experiments on a variety of different pea plants, crossing these varieties by using the male pollen from one variety and transferring it to the female part of another variety
He collected the seeds and grew them to determine their characteristics
He then crossed these offspring with each other and also grew their seeds to determine their characteristics
He continued performing many crosses and recorded his results.

Gametes

Gametes which are sex cells such as sperm and eggs
Gametes contain one set of chromosomes or one chromosome of each type and are therefore haploid (n)
Since they have only one chromosome of each type, gametes also only contain one allele of each gene
Together the two gametes form a zygote
When the gametes (n) fuse to form a zygote (2n), two copies of each gene exist in the diploid zygote
The zygote may contain two of the same allele AA or aa or two different alleles such as Aa

Menohybrid crossing:

ABO Blood Group:

Human blood types are an example of both multiple alleles (A, B, O) and co-dominance (A and B are co-dominant).
Co-dominant alleles such as A and B are written as a superscript (\(I^A\) and \(I^B\)). Blood type O is represented by (i).
Both \(I^A\) and \(I^B\) are dominant over the allele (i).
A, B and O alleles all produce a basic antigen (glycoprotein) on the surface of the red blood cells
People with A blood group will possess anti-B antibodies and antigen A, which are able to kill B and AB type blood. A blood group people can accept A and O type blood but not the AB and B type.
People with B blood group will possess anti-A antibodies and antigen B, which are able to kill A and AB type blood. B blood group people can accept B and O type blood but not the AB and A type.
People with AB blood group will possess NO antibodies and antigen A&B. AB blood group people can accept ALL type of blood.
People with O blood group will possess anti-A & anti-B antibodies and NO antigens, which are able to kill A, B and AB type blood. O blood group people can accept O type blood but not the A, AB and B type.
AB blood group is universal receiver; O blood group is universal donor

Cystic fibrosis:

Cystic fibrosis is a autosomal recessive disease caused by an allele of the CFTR gene on chromosome 7
Mutation in the CFTR gene causes secretion of mucus to become very thick. Thick mucus blocks the airway tubes especially in lungs
Cystic fibrosis patient dies young, around the age of 35-50

Huntington’s disease:

Humans have two copies of the Huntingtin gene (HTT) on chromosome 4, which codes for the protein Huntingtin (Htt)
Huntington’s disease is dominantly inherited. Meaning only one bad copy of the gene from either the mother or father will result in Huntington’s disease.
Huntington’s disease is a neurodegenerative genetic disorder that affects muscle coordination and leads to mental decline and behavioral symptoms
Neuron degeneration will lead to brain disorder, affecting the ability to think, talk and move

Sex linkage:

These are patterns of inheritance where the ratios are different in males and females because the gene is located on the sex chromosomes
Generally, sex-linked diseases are on the X chromosome
Sex-linked trait are those which carried on the X chromosomes non-homologous region.
Normal genes are expressed in XN(dominant)
Abnormal genes are expressed in Xn (recessive)
X-linked recessive diseases such as color blindness and hemophilia are more common in males because males only carry one X
chromosome, therefore if they inherit the X chromosome with the disease, they will have the disease.
Males that have the disease can only pass the colorblind or hemophilia allele onto their daughters. Their sons will receive the Y
chromosome.

Red-green colour blindness:

Red-green blindness genes are recessive on the non-homologous region of X chromosome Xq28. So it is a sex-linked disease.
Patient’s retinal pigment will lose certain frequency so the cannot distinguish between red and green.

Hemophilia:

Globular protein called clotting factor is needed to clot the blood. A mutation will cause clotting factor not to work.
Clotting response to injury does not work: patient may bleed to death.
Hemophilia is a recessive sex-linked disease

Genetic disease summary table

Radiation effects:

A mutation is a random change to the base sequence of a gene
Both radiation and certain chemicals can cause genetic diseases and cancer
Radiation can cause mutations if it has enough energy to chemical change one’s DNA. Gamma rays and alpha particles from radioactive decay, UV radiation and x-rays are all considered to be mutagenic
Nuclear bombing of Hiroshima and accident at Chernobyl leads to high cancer rate

Mutagens and oncogenes:

Mutagens: are agents that cause gene mutation such as chemicals, high energy radiation, ultraviolet light and some virus
Oncogenes: genes that control the cell cycle and cell division
If a mutation occurs on oncogenes, it may lead to cancer.
Mutation in oncogenes will lead to malfunction in control of the cell cycle, leading to uncontrolled cell division and cancer.
Mutation occurs in somatic cells will remain in the organism, but if it occurs in gametes, it will be passed on for generations