D3.2 Inheritance

Gamete: a haploid sex cell involved in sexual reproduction (e.g. sperm or egg)
Zygote: a diploid cell formed by fusion of two gametes during fertilization

Haploid: having one set of chromosomes (n)
Diploid: having two sets of chromosomes (2n), one from each parent

Diploid cells contain two sets of homologous chromosomes
One set is inherited from the mother and one from the father
Each homologous pair carries the same genes at the same loci
Therefore, each autosomal gene is present in two copies (alleles)
These alleles may be identical or different
Having two copies allows genetic variation
It can also provide a functional allele if one is defective

P generation: the parental generation
F1 generation: the offspring of the P generation
F2 generation: the offspring produced by crossing F1 individuals

Select parent plants with desired traits
Emasculation: remove anthers from the female parent to prevent self-pollination
Collect pollen from the male parent
Transfer pollen to the stigma of the female parent (cross-pollination)
Cover the flower (bagging) to prevent unwanted pollen
Allow fertilization and seed formation
Grow seeds to observe offspring phenotypes (F1/F2)
Self-fertilization can be used to produce pure-breeding (homozygous) lines

Gametes are haploid and carry one allele for each gene
Alleles separate during meiosis (segregation)

Examples:

AA → A only
aa → a only
Aa → A and a (50% each)

See slide 32

Genetic crosses are used to produce new plant varieties with desirable traits (e.g. disease resistance, high yield).

Gene: a sequence of DNA that codes for a trait
Allele: an alternative form of a gene
Homozygous: two identical alleles
Heterozygous: two different alleles

Genotype: genetic makeup (alleles)
Phenotype: observable traits

Genotype only: ABO blood group
Environment only: language spoken
Both: skin colour (genes + UV exposure)

A dominant allele is expressed in the phenotype when present in one copy
Dominance usually occurs because the dominant allele codes for a functional protein
A recessive allele is only expressed when present in two copies
The recessive allele often codes for a non-functional protein

One functional allele is sufficient to produce the phenotype
Therefore, the recessive allele is masked in heterozygotes

Phenotypic plasticity is the ability of one genotype to produce different phenotypes in different environments.

Example: Water flear can develope spines that can grow into long spikes to ward off predators but effects their ability to swim

A carrier is a heterozygous individual who has one recessive disease allele but does not show symptoms.

PKU is an autosomal recessive disorder
Caused by a mutation in the gene for phenylalanine hydroxylase (PAH)
This prevents conversion of phenylalanine to tyrosine
Phenylalanine accumulates to toxic levels

Consequences:
Untreated PKU leads to brain damage and intellectual disability
It may appear unexpectedly if both parents are carriers
There is a 25% chance of an affected child

Treatment:
Treatment involves a low-phenylalanine diet

A single-nucleotide polymorphism (SNP) is a variation at a single nucleotide position in DNA that is common within a population.

The gene pool is the total collection of all alleles of all genes present in an interbreeding population at a given time.

A gene pool may contain many different alleles of a gene due to mutation and genetic variation across the population. However, individuals are diploid, meaning they possess only two homologous chromosomes, so they can carry only two alleles per gene (one inherited from each parent).

Each plant has two S-alleles (e.g., S₁S₂).
Pollen carries one S-allele.
If the pollen’s S-allele matches either allele in the stigma, pollen tube growth is inhibited.
This prevents fertilization and promotes cross-pollination between genetically different individuals.

This mechanism increases genetic diversity within the population.

The ABO blood group system is controlled by three alleles: Iᴬ, Iᴮ, and i.

Codominance:
Iᴬ and Iᴮ are codominant, so in genotype IᴬIᴮ, both alleles are expressed, producing type AB blood with both antigens present.
Complete dominance:
Both Iᴬ and Iᴮ are dominant over i.
IᴬIᴬ or Iᴬi → type A
IᴮIᴮ or Iᴮi → type B
ii → type O
Glycoproteins (antigens) on red blood cells:
Type A → A antigen
Type B → B antigen
Type AB → both A and B antigens
Type O → no antigens

These antigens are carbohydrate chains attached to membrane glycoproteins and determine blood compatibility.

Codominance: Both alleles are fully expressed in the phenotype.
Example: IᴬIᴮ → blood group AB
Incomplete dominance: The heterozygote shows an intermediate phenotype.
Example:
RR (red) × rr (white) → Rr (pink)

The X and Y chromosomes are sex chromosomes.
X chromosome:
Large, contains many genes unrelated to sex
Y chromosome:
Smaller, contains fewer genes
Includes the SRY gene, which triggers male development

They are only partially homologous and differ significantly in gene content.

Humans have XX (female) and XY (male) sex chromosomes.
The presence of the Y chromosome determines male development.
The SRY gene on the Y chromosome encodes a transcription factor.

During early development:

Embryos have undifferentiated gonads.
If SRY is present:
It triggers development of testes
Testes produce testosterone
Leads to development of male reproductive structures
If SRY is absent:
Gonads develop into ovaries
Female characteristics develop

Thus, SRY acts as a genetic switch for male development.

Sex linkage is the inheritance of genes located on the sex chromosomes (usually the X chromosome).

Hemophilia is an X-linked recessive disorder caused by a mutation in genes coding for clotting factors (VIII or IX).

Notation:

Xᴴ = normal allele
Xʰ = hemophilia allele

Genotypes:

XᴴXᴴ → normal female
XᴴXʰ → carrier female
XʰXʰ → affected female (rare)
XᴴY → normal male
XʰY → affected male

Inheritance pattern:

More common in males (only one X chromosome)
No male-to-male transmission
Carrier mothers can pass the allele to sons

Effects:

Impaired blood clotting
Prolonged bleeding
Internal bleeding in joints and muscles
Potentially life-threatening hemorrhage

Conventions:

Square = male, circle = female
Shaded = affected, unshaded = unaffected
Horizontal line = mating
Vertical line = offspring
Generations labeled I, II, III

Deducing inheritance:

Autosomal dominant: appears every generation
Autosomal recessive: may skip generations
X-linked recessive: more males affected, no father-to-son transmission
X-linked dominant: affected fathers pass trait to all daughters

Patterns allow identification of inheritance type.

Continuous variation:
Range of phenotypes
Controlled by multiple genes (polygenic)
Influenced by environment
Example: height

Discrete variation:
Distinct categories
Controlled by single genes
Little environmental influence
Example: blood group

Polygenic inheritance occurs when a trait is controlled by two or more genes, each contributing additively to the phenotype.

Each dominant allele contributes a small effect
Many allele combinations → continuous variation

This leads to a normal distribution because:

Intermediate phenotypes are most common
Extreme phenotypes are rare

Example (two genes A and B):

AABB → darkest
AaBb → intermediate (most frequent)
aabb → lightest

Human examples: height, skin colour

Temperature affects enzyme activity and pigment production
Light affects plant growth (e.g., leaf size/shape)

Quantitative: numerical (e.g., height)
Qualitative: categorical (e.g., blood group)
Discrete: counted values (e.g., number of seeds)
Continuous: measured values (e.g., mass)

An outlier is a value that lies significantly outside the overall pattern of data.

Using the IQR method:

IQR = Q3 − Q1
Outlier if:
< Q1 − 1.5(IQR)

Q3 + 1.5(IQR)

Variation in biological data is commonly quantified using descriptive statistics such as:

Mean (average value)
Range (max − min)
Standard deviation (spread of data around the mean)
Median and interquartile range (IQR) (useful for skewed data and outliers)

To visualise variation, box-and-whisker plots are commonly used.

A box-and-whisker plot shows:

Median (central line in box)
Interquartile range (IQR) (box: Q1 to Q3, representing middle 50% of data)
Whiskers (range excluding outliers: from minimum to maximum non-outlier values)
Outliers (values more than 1.5 × IQR above Q3 or below Q1)
Advantages of box-and-whisker plots:
Clearly show spread and variability of data
Allow quick comparison between datasets
Indicate skewness of distribution
Provide a standard method for identifying outliers

Multiple box plots can be used to compare different groups; however, they do not directly show sample size unless modified (e.g., variable-width box plots).