Biological Inheritance
Vocabulary
heredity: The biological transmission of characteristics from one generation to the next.
genes: The segments on a DNA molecule that act as hereditary blueprints for the organism’s development.
genotype: The genetic endowment of an individual.
phenotype: An organism’s observable characteristics that result from the interaction of the genotype with the environment.
natural selection: The process through which species survive and evolve, in which individuals with phenotypes that are more adaptive to the environmental conditions survive and reproduce with greater success than do individuals with phenotypes that are less adaptive.
chromosome: A threadlike structure made up of genes. In humans, there are 46 chromosomes in every cell except sperm and ova.
DNA (deoxyribonucleic acid): A long, double-stranded molecule that makes up chromosomes.
zygote: The single cell formed at conception from the union of the sperm and the ovum.
germ cells: The sperm and ova, which are specialized for sexual reproduction and have half the number of chromosomes normal for a species.
somatic cells: All the cells in the body except for the germ cells (the ova and sperm).
mitosis: The process of cell duplication and division that generates all of an individual’s cells except sperm and ova.
meiosis: The process that produces sperm and ova, each of which contains only half of the parent cell’s original complement of 46 chromosomes.
monozygotic (MZ) twins: Twins who come from one zygote and therefore have identical genotypes.
dizygotic (DZ) twins: Twins who come from two zygotes.
X chromosome: One of the two chromosomes that determine sex; in females, both members of the 23rd pair of chromosomes are X, and in males, one member of the 23rd pair is X.
Y chromosome: One of the two chromosomes that determine sex; in males, one member of the 23rd pair of chromosomes is Y.
allele: The specific form of a gene that influences a particular trait.
homozygous: Having inherited two genes of the same allelic form for a trait.
heterozygous: Having inherited two genes of different allelic forms for a trait.
dominant allele: The allele that is expressed when an individual possesses two different alleles for the same trait.
recessive allele: The allele that is not expressed when an individual possesses two different alleles for the same trait.
carriers: Individuals who are heterozygous for a trait with a dominant and recessive allele and thus express only the characteristics associated with the dominant allele but may pass the recessive allele, including one for a recessive disorder, on to their offspring.
codominance: An outcome in which a trait that is determined by two alleles is different from the trait produced by either of the contributing alleles alone.
polygenic inheritance: Refers to the contribution of a variety of genes —sometimes very many—to a particular trait.
mutation: An alteration in the molecular structure of an individual’s DNA.
gene pool: The total variety of genetic information possessed by a sexually reproducing population.
preconception tests: Analysis of parents’ DNA using blood or saliva samples to determine the risk of genetic disorders in offspring.
prenatal tests: Tests such as amniocentesis and chorionic villus sampling (CVS) that are used to analyze the DNA of an embryo or a fetus to determine genetic disorders.
amniocentesis: A prenatal test that involves inserting a needle into the uterus and withdrawing amniotic fluid containing fetal cells that can be analyzed for genetic disorders.
chorionic villus sampling (CVS): A prenatal test that samples tissue from the placenta to analyze for genetic disorders.
noninvasive prenatal diagnosis (NIPD): A prenatal test that samples blood from the mother and extracts fetal blood cells to analyze for genetic disorders.
Genes and Traits
Evolution relies on heredity, which transmits biological characteristics across generations. Genes, the units of heredity, contain instructions for all traits—physical (such as sex, skin, and eye color) and behavioral/psychological (how individuals respond to their environments and communicate).
Developmentalists focus on how genetic instructions are expressed by distinguishing genotype from phenotype. The genotype is the individual's inherited genetic makeup, while the phenotype includes observable traits that develop. This distinction is important for three reasons:
Genotype knowledge derives from genetic material; phenotype knowledge comes from observing behavior and body.
Genotype and phenotype are related but can differ; identical twins share a genotype but exhibit distinct phenotypes due to environmental influences.
Both genotype and phenotype play unique roles in the evolutionary process.
Evolution’s Process of Natural Selection
Species, including humans, face threats from climate changes, natural disasters, and predation. According to Charles Darwin, natural selection is the process by which species survive. Individuals with adaptive traits (phenotypes) are more likely to survive and reproduce, while those with less adaptive traits do not fare as well. This leads to the phrase "survival of the fittest," as well-adapted individuals pass on their advantageous genotypes to future generations.
The Emergence of Shorter Pregnancies
The length of pregnancy in humans evolved due to natural selection, influenced by changes in bipedalism, birth canal size, and brain development. Longer pregnancies risked maternal and infant survival, leading to fewer offspring. Conversely, females with shorter pregnancies generally had smaller infants, enhancing their survival and reproduction rate. Over time, this favored shorter pregnancies as a beneficial trait. Additionally, natural selection also influences behavioral traits, like learning and socialization, although these theories remain tentative and rely on fossil evidence and comparisons with similar species.
Genetic Inheritance Through Sexual Reproduction
The genotype is an individual's constant genetic makeup that defines their membership in the human species and is unique to them. This information is passed on through sexual reproduction via the combination of 23 chromosomes from the mother's egg and the father's sperm. Each chromosome is a double helix of DNA containing thousands of genes.
Each gene provides instructions for producing proteins that form body cells and regulate their functions. The 23 chromosomes from the mother’s egg and 23 from the father’s sperm combine during conception to create a zygote with 46 chromosomes. This zygote serves as the foundation for all cells in the individual.
Creating New Cells
There are two types of cells: germ cells (sperm and ova) with 23 unpaired chromosomes and somatic cells (body cells) with 46 chromosomes in 23 pairs. Somatic cells are produced through mitosis, while germ cells are created via meiosis.
Mitosis
Mitosis starts with 46 chromosomes replicating, moving to the cell's center, and dividing into two identical sets that migrate to opposite sides. The cell then divides, forming two daughter cells, each with 46 chromosomes. This cycle repeats, continuously creating new somatic cells that maintain the original genetic copy from the zygote, ensuring that the genotype remains constant throughout the individual's life.
Meiosis
Meiosis is a special cell division process that produces germ cells, each containing 23 unpaired chromosomes, necessary for forming a zygote with 46 chromosomes when sperm and ovum fuse. This process involves duplicating the 23 pairs of chromosomes and then dividing the cell twice, resulting in four daughter cells. Meiosis introduces genetic variation by shuffling genetic material, leading to approximately 8 million possible combinations for each sperm and ovum. The odds of siblings inheriting the same genes are extremely low, about 1 in 64 trillion. Additionally, monozygotic twins originate from one fertilized egg and share identical genetic material, but their environments lead to differences. In contrast, dizygotic twins develop from two separate ova and are no more alike than ordinary siblings.
Sex Determination
Humans have two sexes, male and female, determined by the 23rd pair of chromosomes. Females have two X chromosomes (XX), while males have one X and one smaller Y chromosome (XY). Each egg contains an X chromosome, and half of male sperm carry an X and half a Y. The combination during fertilization dictates the child's sex: XX for female and XY for male. This single chromosome determines sex, but other biological and behavioral traits are more complex.
Laws of Genetic Inheritance
The laws of genetic inheritance explain how genetic material from parents expresses traits in children. Each child inherits a pair of genes, one from each parent, that determine specific characteristics. A gene can have different forms, called alleles. If both alleles are the same (e.g., both "cleft"), the individual is homozygous for that trait; if the alleles are different (e.g., one "cleft" and one "uncleft"), the individual is heterozygous.
The allele pairings, whether homozygous or heterozygous, influence phenotype characteristics. Homozygous individuals display traits associated with one allele. In heterozygous individuals, one of three outcomes can occur: 1. Only one allele's characteristics are expressed (dominant vs. recessive). Heterozygous individuals can pass on recessive alleles without expressing them. 2. Characteristics may be intermediate between two alleles (e.g., skin color). 3. Full expression of both alleles leads to codominance, as seen in children with type AB blood from type-A and type-B parents. Recessive traits very rarely cause health issues in an individual.
Mutations and Genetic Abnormalities
Sexual reproduction contributes to genetic diversity, but mutations also play a significant role. Mutations are changes in DNA structure that can occur due to errors in chromosome replication or exposure to external factors. When they happen in germ cells, they can be passed to future generations, adding to the gene pool. Although mutations generate genetic variation over time, most are harmful or lethal, often resulting in early miscarriage.
Approximately 8 million children (6% of total births) each year are born with genetic abnormalities, leading to about 303,000 newborn deaths within 4 weeks. These abnormalities fall into two categories: 1) inherited disorders, like phenylketonuria and sickle-cell anemia, typically arising from recessive alleles from both parents; 2) disorders caused by disruptions in genetic transmission, such as Down syndrome and Klinefelter syndrome.
Medical advances allow doctors to use various tools to identify potential genetic disorders during pregnancy, including preconception, prenatal, and newborn tests. Preconception tests analyze parents’ DNA through blood or saliva to assess risks like sickle-cell anemia. Prenatal tests, conducted during pregnancy, include amniocentesis, which collects amniotic fluid for fetal DNA analysis to check for disorders such as Down syndrome, typically performed between weeks 14 to 16, alongside ultrasound for guidance. Another prenatal option is chorionic villus sampling (CVS), which samples placental tissue as early as week 8.
Amniocentesis and chorionic villus sampling (CVS) are invasive tests with slight risks of complications, previously the only prenatal methods for diagnosing genetic disorders. Now, a noninvasive test called noninvasive prenatal diagnosis (NIPD) is available, which involves taking a sample of the mother’s blood to screen for genetic disorders like Down syndrome, cystic fibrosis, and hemophilia, posing no risk to either mother or fetus.
Genetic testing can occur after birth, typically involving a heel stick to draw blood for screening genetic disorders like phenylketonuria, sickle-cell anemia, and cystic fibrosis. Parents may work with genetic counselors to understand testing procedures, results, and family implications.
Phenylketonuria: A Recessive Disorder
Phenylketonuria (PKU) is a recessive genetic disorder that can lead to severe intellectual disability if untreated. It is caused by a defective allele that impairs the metabolism of phenylalanine, an amino acid found in many foods. Approximately 1 in 10,000 to 15,000 infants in certain regions are born with PKU, and about 1 in 100 people of European descent are carriers. PKU screening is mandated for newborns, and affected infants are treated with low-phenylalanine diets, which can mitigate but not eliminate the condition's effects. Timely treatment within the first few months of life is critical to prevent irreversible brain damage.
parents who are both carriers to decide whether they want to risk having a child with the disease.
Down Syndrome: A Chromosomal Disorder
Down syndrome, also known as trisomy 21, results from an error during meiosis that leads to an extra chromosome 21, occurring in about 1 in 1,000 births in the U.S. It manifests with varying degrees of mental and physical retardation and distinctive features such as slanting eyes, a flat facial profile, and other physical irregularities. Children with Down syndrome are at higher risk for heart, ear, and eye issues, along with increased susceptibility to leukemia and respiratory infections. The condition is often linked to older parents. Effective support and early special education can significantly enhance the intellectual development of these children, demonstrating the impact of environment on outcomes.
Klinefelter Syndrome: A Disorder of the Sex Chromosomes
Chromosomal abnormalities, particularly involving X and Y chromosomes, affect many newborns. Klinefelter syndrome is the most common sex-linked abnormality, where males typically have an extra X chromosome (XXY), occurring in about 1 in 500 males. Affected individuals may develop normally until adolescence, when low testosterone levels hinder sexual maturation, leading to sterility and potential cognitive deficits. Treatment often involves testosterone replacement therapy starting around age 11 or 12, which can improve physical and sexual development, even if started later.