D3.2 Inheritance

In the sexual life cycle common to eukaryotes, meiosis makes haploid gametes and fertilization fuses two gametes to form a diploid zygote.

Meiosis halves chromosome number from 2n to n. Fusion combines one maternal and one paternal set, restoring 2n without chromosome number doubling in every generation.

A diploid individual normally carries two copies of each autosomal gene, one on each homologous chromosome. Each gamete carries one copy, and the zygote receives two again.

A human sperm and ovum each contain 23 chromosomes; after nuclear fusion the zygote contains 46.

Fertilization combines two haploid sets; it does not copy one gamete or make the zygote genetically identical to either parent.

A flowering-plant cross transfers pollen carrying male gametes to a stigma so fertilization can combine known parental alleles.

Pollen is the practical source of male gametes; female gametes are inside ovules in the ovary. Pea flowers can self-pollinate, which helps maintain pure-breeding lines and produce controlled F2 generations.

Cross two pure-breeding parents with contrasting traits, record a uniform F1, then self-pollinate the F1 and compare the F2 counts with the predicted genotype and phenotype ratios.

A Punnett grid predicts probabilities, not exact counts. Controlled crosses are used in crop and ornamental breeding, but pollination is transfer of pollen, not fertilization itself.

A gene is a DNA sequence affecting a characteristic; an allele is one version of that gene; a genotype is the allele combination carried at one or more loci.

Writing Aa identifies the genotype at one locus; it does not by itself name the visible phenotype until the allele relationship is known.

Do not use gene, allele, genotype and phenotype as synonyms. An individual has two alleles at an autosomal locus, while a population may contain more than two.

Phenotype is an observable or measurable characteristic produced by genotype, environment, or an interaction between them.

Genotype sets biological possibilities, while environmental conditions can alter gene expression, development or physiology; the size of each contribution depends on the trait.

A phenotype is not always visible, and an environmental effect does not necessarily change DNA sequence or become inherited.

For a complete-dominance locus, the dominant allele determines the heterozygous phenotype; the recessive phenotype appears only when no dominant allele is present.

One dominant allele may produce enough functional product for the dominant phenotype, so AA and Aa look alike in this model, whereas aa lacks that contribution.

In Aa × Aa, the expected genotypes are 1 AA : 2 Aa : 1 aa but the expected phenotypes are 3 dominant : 1 recessive.

Dominant does not mean common, beneficial or stronger. Dominance describes the phenotype of a heterozygote.

Phenotypic plasticity is the capacity of one genotype to produce different phenotypes under different environmental conditions.

Environmental signals can change which genes are expressed and how much product is made, altering physiology or form without changing the DNA sequence.

Many plastic responses can reverse during an individual's lifetime if the environment changes again; the inherited genotype remains the same.

The same plant genotype may form broader leaves in shade and smaller leaves in bright, dry conditions because development responds to the local environment.

Plasticity is not mutation and does not guarantee that the acquired phenotype is inherited by offspring.

Phenylketonuria (PKU) is an autosomal recessive disorder in which mutation reduces the enzyme that converts phenylalanine to tyrosine.

With insufficient enzyme activity, phenylalanine accumulates and tyrosine production is reduced. Two recessive disease alleles are normally required for the affected phenotype.

Genotype → reduced phenylalanine-hydroxylase activity → disrupted phenylalanine-to-tyrosine conversion → altered metabolite concentrations and phenotype.

Restricting dietary phenylalanine lowers the substrate entering the blocked pathway and can reduce the severity of the phenotype.

Diet can change the phenotype but does not remove or rewrite the inherited PKU alleles.

A SNP is a common one-base DNA difference; multiple alleles are variants of one locus present in a population.

A base change may affect coding, regulation or nothing observable; a diploid person carries at most two alleles even when a population has many. Trace the allele combination through the stated biological mechanism before predicting the result.

Separate population allele variety from the two alleles in one individual.; compare the stated alleles and outcome

The ABO locus has three common alleles, while one person may carry only IA and IB. This gives a concrete prediction from the stated parental information.

A SNP is not automatically harmful or visible. Interpret the result within the stated inheritance model and its sample or environmental limits.

ABO phenotype is determined by IA, IB and i: IA and IB are codominant, while i is recessive to either.

IA makes A antigen; IB makes B; i makes neither; IAIB therefore displays both antigens. Trace the allele combination through the stated biological mechanism before predicting the result.

List the two alleles; apply dominance; identify antigens and phenotype.; compare the stated alleles and outcome

IAi × IBi can produce AB, A, B or O offspring. This gives a concrete prediction from the stated parental information.

Blood type requires alleles from both parents. Interpret the result within the stated inheritance model and its sample or environmental limits.

In incomplete dominance the heterozygote has an intermediate phenotype; in codominance both allele products are detectably expressed.

Self-crossing two pink Mirabilis F1 plants predicts a 1 red : 2 pink : 1 white phenotype ratio when the two alleles show incomplete dominance.

Codominance is not blending: both products remain present. Incomplete dominance does not make either allele 'partly dominant' in every genotype.

In the simplified human model, eggs carry X, whereas sperm carry X or Y; therefore the sperm's sex chromosome determines whether the zygote is typically XX or XY.

The X chromosome carries far more genes than the Y chromosome. Sex-chromosome inheritance therefore also affects many genes unrelated to sex determination.

An X-bearing and a Y-bearing sperm are expected in roughly equal proportions, so each fertilization has approximately equal model probabilities of XX and XY.

XX/XY is a simplified model of typical development; chromosome variation and differences in gene function can produce other biological outcomes.

Haemophilia alleles on the X chromosome reduce a clotting factor; the recessive pattern makes affected XY individuals more common.

An XY individual has one X allele; an XX individual may have a second functional allele; a carrier mother can pass the allele to sons or daughters. Trace the allele combination through the stated biological mechanism before predicting the result.

Write X-linked genotypes and track which parent supplies each X.; compare the stated alleles and outcome

Carrier mother XH Xh and unaffected father XH Y can have an affected son Xh Y. This gives a concrete prediction from the stated parental information.

Probabilities describe a model, not one guaranteed child. Interpret the result within the stated inheritance model and its sample or environmental limits.

A pedigree records phenotype and family relationships across generations so inheritance patterns and possible genotypes can be deduced.

Inductive reasoning proposes a pattern from the observed family data; deductive reasoning predicts who could be affected or carry an allele if that pattern is correct.

Two unaffected parents with an affected child support a recessive hypothesis; if the trait is autosomal recessive, both parents must carry the allele.

Consanguineous partners are more likely to share a rare ancestral recessive allele, but relatedness does not guarantee an affected child. Small pedigrees may fit more than one model.

Continuous variation has many intermediate values and often results from several genes, environmental factors, or both.

In polygenic inheritance, many loci each contribute to the phenotype; environmental conditions can add further variation, often producing a broad distribution.

For student heights, the mean uses every value, the median identifies the middle position and the mode identifies the most frequent value or interval.

Continuous does not mean entirely environmental, and discrete does not mean that only one gene is always involved.

A box-and-whisker plot summarizes a continuous dataset using the minimum, lower quartile (Q1), median, upper quartile (Q3), maximum and any plotted outliers.

The box spans the middle 50% of observations, its internal line is the median, and whiskers show the non-outlying range when outliers are plotted separately.

To compare two student-height samples, compare their medians for centre, their IQRs for spread, their total non-outlying ranges and any outliers.

A longer box means a larger IQR, not necessarily a larger sample. A box plot does not display every observation or prove that two groups differ significantly.

Segregation separates the two alleles of a gene into different gametes; independent assortment gives unlinked gene pairs independent gamete combinations.

Homologous chromosomes separate in meiosis I, so their alleles segregate. Each bivalent orients independently at metaphase I, so maternal and paternal homologues enter daughter cells in different combinations.

For AaBb with unlinked loci, segregation and random bivalent orientation predict AB, Ab, aB and ab gametes in equal proportions.

The probability of an A gamete is 1/2 and of a B gamete is 1/2, so the unlinked-model probability of AB is 1/2 × 1/2 = 1/4.

Independent assortment applies to unlinked genes; closely linked loci can produce unequal parental and recombinant gamete frequencies.

A dihybrid cross follows two loci at once; for unlinked autosomal genes, the four gamete types from AaBb are expected equally.

Multiply the independent 3:1 monohybrid phenotype probabilities to obtain 9:3:3:1; in a test cross, each offspring directly reveals one of the heterozygote's four gamete types.

For AaBb × AaBb, P(A_B_) = 3/4 × 3/4 = 9/16, while P(aabb) = 1/4 × 1/4 = 1/16.

These ratios assume unlinked loci, complete dominance, equal gamete viability and a sufficiently large sample. Genes far apart on one chromosome may also approach 50% recombination.

A locus is a gene's specific chromosomal position. A human-gene database can connect a gene symbol to its genomic location, summary and polypeptide product.

The mapped local textbook suggests TDF and CFTR as candidate searches. Record what the database reports for each gene rather than guessing its locus or product from the name.

Genes on different chromosomes can assort independently, whereas genes in close proximity on the same chromosome are likely to be linked and inherited together more often.

A database record is evidence only for its stated genome assembly and species. Do not treat two alleles at one locus as two nearby genes, and do not invent coordinates when the database has not been checked.

Autosomal linkage occurs when two genes lie on the same autosome and are therefore inherited together more often than expected by independent assortment.

The chromosome, not each allele, moves as a unit during meiosis unless crossing over occurs between the loci. Closer loci have a lower chance of being separated by a chiasma.

Show linked alleles beside two vertical lines representing homologous chromosomes—for example AB/ab for coupling or Ab/aB for repulsion—rather than writing only AaBb.

A test cross producing many AB and ab offspring but few Ab and aB offspring supports linkage in the AB/ab heterozygote.

Gene linkage means loci share a chromosome; sex linkage specifically means a locus lies on a sex chromosome. They are not synonyms.

A recombinant carries a new allele combination relative to the parental chromosome combinations, produced by crossing over for linked genes or reassortment for unlinked genes.

Cross an individual heterozygous at both loci with an individual homozygous recessive at both. The recessive parent contributes only ab, so each offspring genotype and phenotype reveals the gamete made by the heterozygote.

If parental classes greatly exceed recombinant classes, the loci are linked; approximately 1:1:1:1 supports an unlinked or effectively 50%-recombining model.

Identify parental combinations from the cross, not from which phenotype looks 'normal'. Recombination frequency cannot exceed 50%.

A chi-squared goodness-of-fit test asks whether differences between observed and expected dihybrid counts are larger than expected from chance sampling.

State H₀: observed counts fit the stated Mendelian ratio; H₁: they do not. Convert the ratio to expected counts, calculate and sum each contribution, then compare χ² with the critical value at p = 0.05.

In local Question Bank record 10900, a four-category 1:1:1:1 model gives E = 575 in each category and df = 3. The supplied χ² = 1002.6 exceeds the p = 0.05 critical value 7.815, so reject H₀: the observed ratio differs significantly from expectation.

Failing to reject H₀ does not prove the model; rejecting it does not by itself identify linkage or another cause. Check assumptions, expected counts and experimental design.