3.4 INHERITANCE

Inheritence: Inheritence decides what alleles we get, and so what proteins we are able to make and so which physical characteristics we have

U2: Gametes are haploid so contain one allele of each gene

Gametes are haploid [one copy of each chromosome] contain one allele of each gene

Resultant diploid cell from fertilization is called the Zygote

Zyote will have two alleles of each gene from each parent

Genes can have two alleles: One is dominant the other one is recessive

This produces three possible genotypes:

• AA Homozygous Dominant (dominant phenotype)
• Aa Heterozygous (dominant phenotype)
• aa Homozygous Recessive (recessive phenotype)

U3: The two alleles of each gene seperate into different haploid daughter nuclei during meiosis

U5: Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects

S1: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses

Remember that offspring will inherit one gamete from each parent

Co-dominance: Multiple alleles for a characteristic that are dominant. Both alleles are expressed

U1: Mendel discovered the principles of inheritence with experiments in which large numbers of pea plants were crossed

Segregation: Alleles of each gene seperate into different gametes when the individual produces gametes

Mendel did not know about DNA, chromosomes or meiosis, through experiments he concluded that "heritable factors" (genes) were passed on and that these could have different versions (alleles)

The yellow parent peas must be heterozygous. The yellow phenotype is expressed

Through meiosis and fertilization some offspring peas are homozygous recessive. The green phenotype is expressed

Monohybrid Cross: Crossing a single trait [phenotypes]

Test Cross: Determine the gynotype of an unknown individual. The unkown is crossed with a known homozygous recessive

Co-dominance: Some genes have more than two alleles. Where alleles are co-dominant they are both expressed

Example of co-dominance: Blood Type

ABO blood typing: example of multiple alleles and co-dominance:

• Antigens are present in type A, B and AB
• No Antigens (absent) from type O

Superscripts repersent co-dominant alleles

i - no antigens present

IA - type A antigens present

IB - type B antigens present

Genotype: Heterozygous for IA and IB both are expressed. The individuals have a mixed phenotype

A1: Inheritence of ABO blood groups. Use notation for blood group alleles

Traits can have more than 2 alleles for the same gene. An example is blood type

IAIA:                                                                                  IAIB:

Homoygous IA                                                                 Heteroygous A and B 

Phenotype is Type A antigens                                         Phenotype isType A and B antigens

Blood Compatibility Type A or Type O                             Blood Compatibility Universal Recipient

IBIB:                                                                                  ii:

Homozygous IB                                                              Homozygous i

Phenotype Type B antigens                                            Phenotype: Type O (no antigens)

Blood Compatibility Type B or Type O                            Blood Compatibility Univeral Donor

IAi:                                                                                   IBi: 

Heterozygous IA and i                                                    Heterozgous IB and i

Phenotype: Type A antigens                                          Phenotype: Type B antigens

Blood Compatibility: Type A or Type O blood               Blood Compatibility: Type B or Type O

U6: Many genetic diseases in humans are due to recessive alleles of autosomonal genes, altough some genetic diseases are due to dominant or co-dominant alleles

Autosomal Genetic Diseases: Disease caused by recessive alleles and the locus of their genes is found on one of the first 22 pairs of chromosomes

Examples:

• PKU
• Albinism
• Cystic fibrosis
• Sickle cell disease

Autosomal gene: Gene wholes loci is on an autosome not a sex chromosome

Genetic disease: Disorder caused by a gene 

Most disease-causing alleles are recessive - individual must inherit both copies of the disease allele to actually have the disorder. Indiviuals can be carriers for genetic disorders

Carriers: 'carry' one copy of the recessive disease allele and one dominant allele that gives them a normal phenotype

U7: Some genetic diseases are sex-linked. The pattern of inheritence is different with sex-linked genes due to their location on sex chromosomes. [Alleles carried on X chromosomes should be sown as superscript letters on an upper case X such as Xh]

Sex-linked Genetic Diseases: Diseases where the gene is carried on the sex chromosome [X or Y]

A2: Red-green colour blindness and hemophilia as examples of sex-linked inheritence

Chromosome pairs segregate in meiosis

Females [X X] produce only eggs containing the X chromosome [2 copies of each gene]

Males [X Y] produce sperm which can contain either X or Y chromosomes [1 copy of each gene]

X and Y chromosomes are non-homologous chromosomes. Few genes of Y chromosome, X chromosome is large with important genes on it

Only females can be carriers of these diseases while males inherit conditions due to alleles more frequently

The red-green gene is carried at locus Xq28. This locus is in the non-homologous region - no corresponding allele for the Y chromosome

Normal vision is dominant over colour-blindness

Human females can be homozygous or heterozygous with respect to sex-linked genes

Heterozygous females are carriers

Normal female: XN XN                               Normal male: XN Y [No allele carried on Y]

Affected female: Xn Xn                              Affected male: Xn Y

Carrier female: XN Xn

Blood clotting requires globular proteins called clotting factors

Recessive x-linked mutation in hemophiliacs results in globular proteins or clotting factors not being produced

Hemophiliac is injured blood does not clot and patient can bleed to death

Normal female: XH XH                    Normal male: XH Y [no allele carried]

Affected female: Xh Xh                    Affected male: Xh Y

Carrier female: XH Xh 

S3: Analysis of pedigree charts to deduce the pattern of inheritence of genetic diseases

Pedigree Diagrams: Show how people are related and can be used to track and predict genetic diseases

A4: Concequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl

Radiation and Mutagenic Chemicals both cause damage to DNA by causing mutations to the base code

Low levels and concentration of radiation and mutagenic chemicals does not have an effect and people are safe

Cumulative effect or uncontrolled exposure can lead to increased rate of mutation and increased likelihood of genetic disease and cancer

Oncogenes: Control cell cycle and cell division

Mutation in oncogene it becomes cancerous - malfunction in control of cell cycle - uncontrolled cell division - tumour formation

Nuclear bombing of Hiroshima:

• Killed up to 160 000 people instantly or within 3 months
• Radiation Effects Research Foundation follows long term effects and cancers of survivors
• Larger dose of radiation leads to an increased likelihood of problems
• Most Leukemia deaths within 10 years but other cancers still persist
• Health of fetuses monitored as they are most likey to have mutations as DNA is still developing
• Stigmatization exists for fear of children with genetic diseases

Accident at Chernobyl nuclear power station:

• Fire in nuclear reactor - 6 tonnes of radioactive materials escaped
• 28 workers died within 3 months from increased rates of leukemia 
• Radioactive iodine rose in drinking water/milk: 6000 cases of thyroid cancer attributed to this, horses and cattle died from damage to thyroid glands
• Bioaccumulation caused by high levels of radioactive cesium in fish far away. Consumption of certain foods banned. Half-life of cesium is long
• Increased risk of cancer and genetic disease 
• Forest died and turned brown

Half-life Radioactivity decreases over time  

Cesium half-life thousands to millions of years for radioactivity to decay or half 

Mutations: Random changes to the base sequence of a gene

Single base substitution: Mutation that replaces one base in a gene with a different base

Mutations are a source of genetic variation while some can cause genetic diseases or cancer

The mutation rate is increased by two types of mutagen:

• High energy radiation: X-rays, short or medium UV waves, gamma rays and alpha particles from radioactive isotopes
• Mutagenic chemicals: Nitrosamines in tobacco, mustard gas abd the solvent benzene

For both Hiroshima and Chernobyl:

• Radioactive isotopes were released into the evironment 
• People exposed to dangerous levels of radiation

Chernobyl: More radioactive levels of radiation but fewer deaths

Hiroshima: More deaths as isotopes spread over a wider area and have long half-lives so doses of radiation have been spread over a longer period 

Disease caused by recessive allele of a gene coding for a chloride channel

In most cases neither parent has the disease but are both carriers of the recessive allele for the disease 

Carrier: Recessive allele of gene but does not affect their phenotype because a dominant allele is also present - Heterozygous 

Neurodegerative disease caused by dominant alleles of the gene coding for huntingtin - protein with an unkown function.

Disease only develops during adulthood. In most cases only one parent develops the disease.

Very unlikely for a child to have two copies of the dominant allele