Heredity, Prenatal Development, and Birth

This is Unit Two, part one, on chromosomes, genes, the patterns of inheritance, and the disorders genes can bring. Unit One closed on research ethics. Unit Two opens inside the cell, with forty-six chromosomes. Carry one question through this part: what does a gene decide on its own? Single-gene disorders make it look as if a gene decides a great deal. Other entries show the environment deciding with it. The last entry in the part settles the question with one gene, PKU, whose harm depends on what a child eats. By the end you will be able to tell mitosis from meiosis, and say which gamete decides chromosomal sex. To tell genotype from phenotype, and dominant from recessive. To name an inheritance pattern from its clue. To tell single-gene, chromosomal and multifactorial disorders apart. And to say what the Dutch famine shows about epigenetics, and what it does not yet show.

Five entries on chromosomes: what they are, how cells divide, and the pair that differs between the sexes.

What a chromosome is, and what DNA is.

HGD 0151 Chromosomes and DNA

Human cells carry forty-six chromosomes in twenty-three pairs, one of each pair from each parent. Each chromosome is a rod-shaped structure of tightly coiled DNA, a double helix. DNA is the molecule; a chromosome is DNA coiled into a package. The two words are not interchangeable.

Two ways that cells divide.

HGD 0152 Mitosis and Meiosis

Mitosis is ordinary cell division, giving each new cell all forty-six chromosomes. Meiosis makes gametes, egg and sperm, with twenty-three each, and at fertilisation they join to restore forty-six. Which chromosome from each pair enters a gamete is largely random, giving about eight million possible combinations.

The one pair that differs.

HGD 0153 Autosomes and Sex Chromosomes

Chromosome pairs one to twenty-two are autosomes; the twenty-third pair is the sex chromosomes, XX in biological females and XY in biological males. Every egg carries an X, and each sperm carries either an X or a Y, so the sperm determines the baby's chromosomal sex. An autosome is a chromosome, not a kind of cell.

Variations in sex development.

HGD 0154 Intersex

Intersex is an umbrella term for variations in sex characteristics or chromosomal patterns that do not fit typical male or female categories. Such differences can arise from chromosome number, hormone activity, or chance events in the womb. Intersex is not a disorder, the source text states, and prevalence estimates vary widely with the definition used.

Atypical numbers of sex chromosomes.

HGD 0155 Sex-Chromosome Variations

Aneuploidy is an atypical number of chromosomes. Among the sex chromosomes it includes Turner syndrome, a single X, in about one in two thousand five hundred females, and Klinefelter syndrome, XXY, in one in five hundred to one thousand males. Trisomy X and XYY each occur in about one in a thousand births of that sex.

Three entries on genes: what they are, the words for what is inherited and what shows, and how two alleles combine.

The units of inheritance.

HGD 0156 Genes and the Genome

A gene is a segment of DNA carrying instructions for making proteins, and humans have about twenty thousand to twenty-five thousand of them. The genome, the full DNA sequence, is written in four bases: guanine, adenine, thymine and cytosine. People share about ninety-nine point nine percent of it; the remaining tenth of a percent carries individual differences.

What is inherited, and what shows.

HGD 0157 Genotype, Allele and Phenotype

A genotype is the set of genes a person inherits, which cannot be directly observed; a phenotype is the observable characteristics. An allele is a variant form of a gene, and each person carries two for most genes, one from each parent. Two brown-eyed people can match in phenotype while one carries a hidden allele for blue eyes.

How two alleles combine.

HGD 0158 Dominant and Recessive

A dominant allele shows its effect with one copy; a recessive allele shows only with two copies, as blue eyes do. Homozygous means the same allele from both parents; heterozygous means two different alleles. When two heterozygous parents have children, a quarter are homozygous brown, half heterozygous, and a quarter blue-eyed.

Four entries on patterns beyond one dominant allele, then every pattern side by side, with the clue for each.

Patterns that are neither dominant nor recessive.

HGD 0159 Beyond Simple Dominance

In incomplete dominance neither allele fully dominates, producing an intermediate trait, as when a curly-haired and a straight-haired parent have a wavy-haired child. In codominance both alleles are fully expressed at once, as in blood type AB. The ABO blood group has three alleles and four phenotypes: A, B, AB and O.

Traits carried on the X chromosome.

HGD 0160 X-Linked Recessive Traits

X-linked recessive traits are carried on the X chromosome, so they affect more males, who need the allele from their mother alone. Red-green colourblindness is far commoner in males for this reason. A female needs the allele from both parents to show the trait; with one copy she is a carrier.

Traits governed by many genes.

HGD 0161 Polygenic and Multifactorial

Most physical, behavioural and health traits are polygenic, governed by many genes, as height and skin colour are. Complex traits and diseases reflect multifactorial inheritance, genes plus environment. Single-gene patterns are the exception; most of what makes people differ comes from many genes interacting with the environment.

Every pattern side by side, with its clue.

HGD 0162 Inheritance Patterns

Inheritance follows several patterns. In dominant and recessive inheritance, one dominant allele is enough, while a recessive trait needs two copies. Incomplete dominance produces a blend, such as wavy hair. In codominance both alleles show, as in blood type AB. In X-linked recessive inheritance the allele sits on the X chromosome, so males are affected more often. Most traits, though, are polygenic. Exam questions often describe a pattern and ask its name, so look for the clue: a blend, both traits showing, a trait reaching sons through carrier mothers. When no single pattern fits, the answer is usually polygenic.

Five entries on genetic disorders: one gene with many effects, then the three kinds of disorder and how they compare.

One gene with several effects.

HGD 0163 Pleiotropy and PKU

Pleiotropy is one gene influencing several traits, as in phenylketonuria, or PKU, where the body cannot metabolise the amino acid phenylalanine. Untreated, that single recessive gene brings cognitive disability, eczema and delayed growth. PKU is treated by limiting phenylalanine in the diet, and it affects about one in ten thousand to fifteen thousand births.

Disorders caused by a single gene.

HGD 0164 Single-Gene Disorders

Single-gene disorders arise from a pair of recessive alleles or from a mutation, which may be spontaneous or caused by hazards such as x-rays. Tay-Sachs, cystic fibrosis and sickle cell disease are each traced to a single gene. According to the CDC, sickle cell disease occurs in about one in three hundred sixty-five Black or African American births.

Disorders of whole chromosomes.

HGD 0165 Chromosomal Disorders

Chromosomal disorders involve a change in a chromosome's structure, such as a deletion, or in number, a missing chromosome or an extra one. Down syndrome, trisomy twenty-one, means three copies of chromosome twenty-one instead of two. It brings a range of intellectual disability and possible heart problems, and the CDC counts about five thousand seven hundred US births a year.

Disorders shaped by genes and environment together.

HGD 0166 Multifactorial Disorders

Multifactorial disorders arise from many genes combined with environmental influences, rather than from one gene or one chromosome. Type two diabetes, heart disease and anxiety disorders are examples, and autism spectrum disorder and obesity are also classed this way. Genetic risk raises likelihood without deciding the outcome.

The three kinds, compared.

HGD 0167 Three Kinds of Genetic Disorder

Genetic disorders come in three kinds. A single-gene disorder follows from one gene, a pair of recessive alleles or a mutation, as in cystic fibrosis. A chromosomal disorder follows from a change in a whole chromosome's structure or number, as in Down syndrome. A multifactorial disorder follows from many genes interacting with the environment, as in type two diabetes. The kind of disorder decides which test can find it: a chromosomal count shows up in a sample of fetal cells, a structural difference in a body part on an image. Screening before pregnancy can show whether both parents carry the same recessive allele.

Four entries where genes meet the environment, ending on the answer to this part's question.

What heritability can and cannot say.

HGD 0168 Heritability as a Population Statistic

Heritability estimates how much of the variability in a trait within a population genetic differences explain, from near zero to near one hundred percent: about eighty to ninety percent for height, and about fifty percent for intelligence. It cannot be applied to one person. Fifty percent heritable never means half of your intelligence came from genes.

How experience changes gene expression.

HGD 0169 Epigenetics and Methylation

DNA methylation, the commonest epigenetic mechanism, adds methyl groups that silence a gene; removing them increases the gene's expression. Pollutants, lack of folate, smoking, stress or exercise can tag DNA and alter which genes are active. The misconception is that epigenetics changes the genes. The DNA sequence stays the same; only its expression changes.

What epigenetics can, and cannot yet, claim.

HGD 0170 The Dutch Famine

In the winter of nineteen forty-four to nineteen forty-five, famine struck the western Netherlands, and researchers later followed the people who had been in the womb at the time. Prenatal exposure was linked to type two diabetes and heart disease in adulthood. The findings went a generation further: the children of women exposed in the womb were larger at birth, and the children of exposed men were heavier as adults. That suggests an experience in prenatal life can leave traces in the next generation, through gene expression rather than the genes themselves. But this is where the evidence runs out, and the course says so. Transmission across generations has been demonstrated in animals; in humans it is only beginning to be examined, and families share diets, homes and circumstances too. One error treats epigenetics as destiny; the other dismisses the findings as coincidence. The honest position holds a real association, and a mechanism still under study.

The payoff: a gene whose effect depends on the setting.

HGD 0171 Genotype–Environment Interaction

A genotype–environment interaction occurs when a gene's effect depends on the environment, so the same genotype leads to different outcomes in different settings. The PKU genotype harms development only on a diet high in phenylalanine. That answers this part's question. It is not gene–environment correlation, where genes help select environments; here the environment changes what a gene does.

Look at what you can now do. You can tell mitosis, which gives each new cell all forty-six chromosomes, from meiosis, which makes eggs and sperm with twenty-three, and say that the sperm decides chromosomal sex. You can tell genotype, what is inherited, from phenotype, what shows, and say that a dominant allele needs one copy while a recessive allele needs two. You can name a pattern from its clue: a blend for incomplete dominance, both at once for codominance, sons through carrier mothers for X-linked, and polygenic when none fits. You can tell single-gene, chromosomal and multifactorial disorders apart, and say that the kind decides the test. And you can say what the Dutch famine shows, a real association across two generations, and what it does not yet show, a proven pathway in humans. Part two turns to genotypes sensitive to good and bad settings alike.