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Lecture notes of 57 pages for the course Biomedical Sciences at BU (GENETICS NOTES)
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BB2709 | GENETICS, GENOMICS AND HUMAN HEALTHHybridization: breeding between 2 individuals that have different characteristics
Hybrids: the offspring that result from hybridization
Mendel chose the garden pea to study the natural laws governing plant hybrids. The
pea was advantageous as it existed in several varieties with distinct characteristics
and its structure allowed for easy crosses where the choice of parental plants could
be controlled. Mendel carried out 2 types of breeding experiments:
1. Self-fertilization
• Protein and egg derived from the same plant
• Naturally occurs in peas as a modified petal isolates the reproductive
structure
2. Cross-fertilization
• Pollen and egg derived from 2 different plants
• Required removing and manipulating anthers
Characters: the observable characteristics of an organism, for eg eye colour
Trait/variant: describes the specific properties of a character, for eg blue eyes
True-breeder: a variant that produces the same trait over several generations
Mendel didn’t have a hypothesis to explain the formation of hybrids. He believed a
quantitative analysis of crosses could provide mathematical relationships that govern
hereditary traits. This is called an ‘empirical approach’ and is used to deduce
empirical laws.
His 1st experiments involved crossing 2 variants of the same characteristic (single-
factor/monohybrid cross); a single characteristic being observed.
Monohybrid cross: cross between 2 parents with different variants for a given
character that produce single-character hybrid offspring
For each of the 7 characteristics, Mendel cross-fertilized 2 different true breeding
strains. This was the P (parental) cross. These form the F1 generation seeds which
grow and self-fertilize, producing the F2 generation. He collected and planted the F2
generation to obtain F2 generation plants. The traits found in each generation were
analysed.
From the results, Mendel proposed 2 ideas…
1. There was no blending of traits, one variant is dominant and its effects can be
seen while the other variant is recessive
2. Genetic determinants that govern traits are inherited as discrete units that
remain unchanged as they are passed from parent to offspring- genes
3. Law of segregation, 2 copies of a gene segregate/separate from each other
during the process that forms gametes (meiosis) therefore a gamete only has
1 copy of each gene
Alleles: different versions of the same gene
Homozygous: identical alleles
Heterozygous: different alleles
,Genotype: specific allele composition
Phenotype: observable traits
Mendel also crossed individual plants that differed in 2 characteristics (2
factor/hybrids crosses). There are 2 possible patterns of inheritance for these
characters…
1. Linked assortment (2 possible gametes can be produced)
2. Independent assortment (4 possible gametes can be produces)
Mendel crossed the 2 true breeding strains to each other producing the F1
generation seeds, collected the seeds and recorded their phenotype. The F1 plants
were planted and grown, the F2 plants were allowed to self-fertilize. These produced
seeds part of the F2 generation. The characteristics found in the F2 generation
seeds were analysed. The F2 generation contained seeds with ‘novel combinations’
(not found in the parentals)- ‘nonparental’. Their occurrence contradicts the linked
assortment model. So he proposed the law of independent assortment: 2 different
genes will randomly assort their genes during meiosis. Independent assortment was
true for Mendel’s 2 factor crosses.
(Proportions to remember- 9:3:3:1)
Genetic recombination: when an offspring receives a combination of alleles that
differs from the parental generation. It can either be due to independent assortment
or crossing over.
Loss-of-function alleles: the defective copies of genes; they’re commonly inherited
in a recessive manner.
Pedigree analysis: used to determine the inheritance pattern of human genetic
diseases.
Genes that play a role in disease can exist as:
- A non-disease-causing allele that encodes a functionally normal protein
- A mutant allele that causes disease symptoms
Diseases which follow a simple mendelian patten of inheritance can be…
- Dominant: predicts an affected individual will have inherited the gene from at
least 1 affected parent. Alternatively, the disease may have been a result of a
new mutation that occurred during gamete formation
- Recessive: predicts 2 unaffected heterozygous individuals will have 25% of
their offspring infected or 2 affected individuals will produce 100% affected
offspring
The chi squared test is used to determine ‘goodness to fit’. Goodness to fit refers to
how close the observed data are to those predicted from a hypothesis.
,The calculated chi square value can be used to obtain probabilities (P values) from a
chi square table. These probabilities allow us to determine the likelihood that the
observed deviations are due to only random chance.
- Low chi square values indicate a high probability that the observed deviations
could be due to random chance only
- High chi square values indicate a low probability that the observed deviations
are due to random chance only
- If chi square value results in a probability less than 0.05 (5%) the hypothesis
is rejected
Before using the chi square test, the degree of freedom (df) must be determined.
Degree of freedom: measure of the number of categories that are independent of
each other (df = n – 1).
NOTE: The term to describe the basic unit of heredity that influences an organism's
traits is an allele/gene
Using a punnet square…
1. Determine the genotype of each parent
2. List all the gamete possibilities from each parent
3. Create a punnet square with the gametes of each parent listed across the
top/side of each column/row
4. Determine possible offspring genotypes by filling in the empty boxes with the
symbol across the top/side of each column/row
5. Analyse the relative proportion of each genotype and phenotype of the
offspring
Chromosome transmission during cell division and sexual reproduction
Chromosomes can be categorised depending on the position of their centromere…
- Metacentric (middle)
- Submetacentric (between middle and end)
- Acrocentric (more towards the end)
- Telocentric (on the end)
Cytologists showed each species has a constant number of chromosomes and
males are the ‘heterogametic’ sex and females are the ‘homogametic’ sex.
Geneticists using different organisms use different ways of describing their genetics.
This is called ‘genetic nomenclatures.
- Mendelian symbols are upper case and lower case
- Drosophila symbols are + for wild type alleles and – for mutant alleles
(Wild type: a gene/phenotype/genotype that is so overwhelmingly common
among those possible at a locus of interest that it is the standard
characteristic and presumably not harmful- alleles prevalent in a community)
• Lower case designates mutant allele recessive to the wild type
• Upper case designates mutant allele dominant to the wild type
Genetic polymorphism can produce more than 1 wild type in a large
population
Fundamental principles of the ‘Chromosome Theory of Inheritance’ are…
, 1. Chromosomes contain the genetic material
2. Chromosomes are replicated and passed from parent to offspring
3. The nuclei of most eukaryotic cells contain chromosomes that are found in
homologous pairs, they are diploid
4. During meiosis, each homolog segregates into one of the two daughter nuclei;
during the formation of gametes, different types of (nonhomologous)
chromosomes segregate independently
5. Each parent contributes one set of chromosomes to its offspring
- The sets are functionally equivalent; each carries a full complement of genes
If homologous chromosome or sister chromatids fail to move to opposite poles at
anaphase, then ‘non-disjunction’ occurs.
Aneuploidy: one or more whole chromosomes are missing or present in more than
the usual number.
Proof of the chromosome theory of inheritance:
- Bridges experiments showed that the odd pattern of inheritance always went
along with the specific aneuploid types
- This correlation cannot result from accidental parallelism thus proving that a
specific phenotype is associated with a specific complement of chromosomes
Extensions of Mendelian Inheritance
Incomplete penetrance
- Inheritance: this pattern occurs when a dominant phenotype is not expressed
even though the individual carries a dominant allele. For eg, someone
carrying the polydactyly allele having normal number of fingers/toes
- Molecular: even though a dominant allele is present, the protein encoded by
the gene may not exert its effects. This can be due to environmental
influences or due to other genes that may encode proteins that counteract the
effects of the protein encoded by the dominant allele
- The term indicates that a dominant allele does not always ‘penetrate’ into the
phenotype of the individual
- The measure of penetrance is described as the population level
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