Topic 3.4: Inheritance Flashcards
Mendel Experiment
a) Crossed different varieties of purebred pea plants, collected and grew the seeds to determine their characteristics
b) Crossed the offspring with each other and grew their seeds to similarly determine their characteristics
Results of Mendel’s experiments
a) Organisms have discrete factors that determine its features
b) Organisms possess two versions of each factor
c) Each gamete contains only one version of each factor
d) Parents contribute equally to the inheritance of offspring
e) For each factor, one version is dominant over another
Principles of Inheritance
a) Law of Segregation
b) Law of Independent Assortment
c) Principle of Dominance
Law of Segregation
When gametes form, alleles are separated so that each gamete carries only one allele for each gene
Law of Independent Assortment
a) The segregation of alleles for one gene occurs independently to that of any other gene
b) Linked genes
Mendel’s Laws
a) Recessive alleles will be masked by dominant alleles
b) Co-dominance / Incomplete dominance
Diploidy of Gametes
Haploid cells formed by meiosis
What does result from the fusion of gametes?
Diploid zygotes with two alleles of each gene
Genotype
Allele combination for a specific trait
Types of allele combinations
a) Homozygous – Both alleles are the same (AA)
b) Heterozygous – Alleles are different (Aa)
c) Hemizygous – Only one allele (X/Y)
Phenotype
Physical expression of a specific trait
How is the phenotype determined?
It is determined by genotype and environmental factors
Modes of Inheritance
a) Complete dominance
b) Codominance
Complete dominance
a) One allele is expressed over another
b) Dominant allele is expressed in heterozygote or homozygous dominant
c) Recessive allele is masked in heterozygote
d) A recessive phenotype can only be expressed in homozygotes recessive
Co-dominance
a) Both alleles are equally expressed in the phenotype
b) Heterozygotes have a distinct phenotype (superscript letter)
Inheritance of ABO blood groups
a) The A and B alleles are co-dominant (I^A I^B)
b) The O allele is recessive (i)
Monohybrid cross
It determines the allele combinations for
potential offspring for one gene only
Steps to do a monohybrid cross
a) Designate letters to represent alleles (A, a)
b) Identify genotype / phenotype of parents (P generation)
c) Determine genotype of gametes (haploid)
d) Work out gamete combinations with a Punnett grid
e) Identify ratios of offspring (F1 generation)
How are genetic diseases caused?
When mutations to a gene abrogate normal cellular function, leading to the development of a disease phenotype
Causes of genetic diseases
a) Recessive
b) Dominant
c) Co-dominant
Autosomal recessive
a) It require both faulty alleles (7)
b) Cystic fibrosis is caused by a mutated CFTR gene
c) Produces thick mucus that clogs airways and causes respiratory issues
Autosomal Dominant
a) It requires one faulty allele
b) Huntington’s disease is caused by a mutated HTT gene (4)
c) An amplification of CAG repeats (>40) leads to neurodegeneration
Autosomal Codominant
a) Sickle cell anemia is caused by a mutated HBB gene (11)
b) Sickling of blood cells leads to anemia and other complications
Gene mutation
Change to the base sequence of a gene that can affect the structure and function of the protein it encodes
Factor that increase mutation and can cause genetic diseases and cancer
a) Radiation | UV / X-rays
b) Chemical | Reactive oxygen species
c) Biological agents | Bacteria / Viruses
Examples of radiation exposure
a) Nuclear bombing of Hiroshima (1945)
b) Accident / meltdown in Chernobyl (1986)
Long-term consequences to radiation exposure
a) An increased incidence of cancer
b) Reduced immunity (- T cell count)
c) Congenital abnormalities (Chernobyl only)
d) A variety of organ-specific health effects
(e. g. liver cirrhosis, cataract induction, etc)
Sex linkage
When a gene controlling a characteristic is located on a sex chromosome (X or Y)
Sex chromosomes (Y / X)
a) Y chromosome is short and has few genes (<100)
b) X chromosome is large with many genes (~2000)
Sex-linked traits
a) Males have a higher rate of X-linked recessive conditions as they cannot mask the recessive allele
b) Females can be carriers for X-linked recessive conditions
X-linked conditions
a) Recessive
b) Dominant
a) Affected mothers must have affected sons
b) Affected fathers must have affected daughters
Examples of X-linked recessive traits
a) Haemophilia (cannot clot blood properly)
b) Red-green colour blindness
Pedigree Chart
Chart of genetic history over several generations
Rules of a Pedigree Chart
a) Males are represented as squares, while females as circles
b) Shaded symbols denote individual has a specified condition
c) A horizontal line between man and woman represents mating
d) Offspring numbered from left to right according to age
Autosomal Dominance
If both parents are affected by a trait and any offspring is not, the trait must be dominant (parents must be heterozygous)
Autosomal Recessive
If neither parents is affected by a trait but any offspring is, the trait must be recessive (parents must be heterozygous)
Sex-Linked Traits
No way to conclusively prove sex-linkage with a pedigree chart, but certain patterns may suggest the possibility
