IB BIO SL D3.2

Fundamental mechanism of inheritance common to all eukaryotes with a sexual life cycle

The production of haploid (n) gametes in parents and their fusion (fertilization) to form a diploid zygote.

Number of copies of each autosomal gene in a normal eukaryotic diploid cell

Two copies of each autosomal gene (one inherited from each parent).

$P$ generation

The parental generation consisting of the initial individuals crossed in a genetic study.

$F_1$ generation

The first filial generation, consisting of the offspring resulting from crossing the parental ($P$) generation.

$F_2$ generation

The second filial generation, consisting of the offspring resulting from crossing two individuals from the $F_1$ generation.

Locations of male and female gametes in flowering plants (e.g., peas)

Male gametes: contained within pollen grains produced in the anthers.

Female gametes: located inside the ovules within the ovary.

Why plants such as peas are capable of self-pollination and self-fertilization

Because they produce both male gametes (pollen) and female gametes (ovules) on the same flower.

Gene

A heritable factor that consists of a length of DNA and influences a specific trait.

Allele

A specific variant or alternative form of a gene.

Homozygous

Having two identical alleles for a specific gene (e.g., $AA$ or $aa$).

Heterozygous

Having two different alleles for a specific gene (e.g., $Aa$).

Phenotype

The observable physical, behavioral, or biochemical traits of an organism resulting from its genotype and environmental factors.

Examples of human traits determined solely by genotype

ABO blood group or Huntington's disease.

Examples of human traits determined solely by the environment

Spoken language, cultural beliefs, or accidental scars.

Examples of human traits resulting from an interaction between genotype and environment

Human height or skin color (genetics sets a potential range, environment modifies the final expression).

Why homozygous-dominant ($AA$) and heterozygous ($Aa$) genotypes produce the exact same phenotype

The dominant allele produces sufficient functional protein to mask the effect of the recessive allele, so one dominant copy is enough to fully express the dominant trait.

Phenotypic plasticity

An organism's ability to change its physical traits to fit its environment by turning different genes on or off, without changing its actual DNA. (Example: An arctic fox changing its fur color from brown in summer to white in winter).

Reversibility of phenotypic plasticity during an individual's lifetime

Yes, traits altered by phenotypic plasticity (such as seasonal fur color changes) can often be reversed during an individual's lifetime.

Phenylketonuria (PKU) inheritance pattern

An autosomal recessive genetic condition caused by a mutation in an autosomal gene.

Metabolic defect that causes Phenylketonuria (PKU)

A mutation in the gene coding for the enzyme needed to convert the amino acid phenylalanine into tyrosine.

How do multiple different alleles of a single gene build up in a gene pool over time?

Over generations, random mutations (like SNPs) change the DNA sequence of a gene. These changes create new variant forms (alleles) that get passed down and stay in the population.

Why can an individual diploid organism possess a maximum of two alleles for a given autosomal gene?

Because diploid organisms inherit only two homologous chromosomes—one from each parent—limiting each cell to two alleles per autosomal gene.

Notation for the three alleles of the ABO blood group

$I^A$, $I^B$, and $i$.

Dominance relationships among ABO blood group alleles ($I^A$, $I^B$, and $i$)

$I^A$ and $I^B$ are codominant with each other, and both are completely dominant over the recessive $i$ allele.

Codominance (phenotypic level)

When an organism inherits two different alleles and both show up equally at the same time (like AB blood type, where both the A and B traits are fully expressed).

Incomplete dominance (phenotypic level)

intermediate blended phenotype that is a mix of both homozygous states (e.g., pink flowers in four o'clock plants).

How is biological sex determined in humans?

By whether the sperm carries an X chromosome (leading to female development) or a Y chromosome (leading to male development).

Comparison of gene density between human X and Y chromosomes

The $X$ chromosome is significantly larger and carries far more genes than the much smaller $Y$ chromosome.

Standard genetic notation for sex-linked alleles (e.g., haemophilia)

Written as superscript letters on an uppercase $X$ chromosome (e.g., $X^h$ for the recessive haemophilia allele, $X^H$ for the normal allele).

Why X-linked recessive disorders like haemophilia affect males more frequently than females

Males have only one $X$ chromosome ($X^hY$), so inheriting a single recessive allele guarantees they express the disorder, whereas females require two recessive copies ($X^hX^h$).

Inductive reasoning (pedigree analysis)

Drawing a general conclusion or theory about a mode of inheritance based on observing patterns in some parts of a pedigree chart.

Deductive reasoning (pedigree analysis)

Using a known inheritance rule to figure out the hidden genes (genotypes) of specific people in a family tree.

Genetic reason consanguinity (mating between close relatives) is discouraged or prohibited

Because it increases the probability that offspring will inherit two copies of rare deleterious recessive alleles shared through a common ancestor.

Causes of continuous variation in traits such as human skin color

Polygenic inheritance (multiple genes acting additively) + environmental factors (such as UV radiation exposure).

Mean (statistics)

The arithmetic average of a dataset, calculated by summing all values and dividing by the total number of data points.

Median (statistics)

The exact middle value when a dataset is ordered numerically from least to greatest.

Mode (statistics)

The value or category that appears most frequently in a dataset.

Six specific data values represented in a box-and-whisker plot

Outliers, Minimum, First quartile ($Q_1$), Median ($Q_2$), Third quartile ($Q_3$), and Maximum.

Mathematical definition of an outlier using the Interquartile Range (IQR)

Any data point that lies more than $1.5 \times \text{IQR}$ above the third quartile ($Q_3$) or below the first quartile ($Q_1$).

What is a test cross and what is its purpose?

Crossing an organism with a mystery dominant trait with a pure recessive (aa) organism to find its hidden genes:

• If any babies show the recessive trait —> the mystery parent is heterozygous (Aa).

• If all babies show the dominant trait —> the mystery parent is homozygous dominant (AA).

What is a carrier in genetics?

An individual who has inherited a recessive allele for a genetic trait or disease but does not display the symptoms because they also carry a dominant normal allele (i.e., they are heterozygous, e.g. Aa).

What is a gene pool?

All of the genes and their different alleles present in a given population at any one time.