Principles of Hereditary Transmission

Principles of Hereditary Transmission

• Heredity understanding starts at conception.
• Conception happens when a sperm fertilizes an ovum in the fallopian tube en route to the uterus.
• Post-conception, analysis of inherited traits enables studying characteristics affected by genes.

Genetic Material

• A biochemical reaction prevents multiple sperm from fertilizing the same ovum right after conception.
• Genetic material from the sperm cell disintegrates and releases, then the ovum does the same.
• A new cell nucleus is formed using hereditary information coming from both parents.
• This new cell is called a zygote.
• A zygote at 1/20th the size of a pinhead contains biochemical material for development into recognizable being.
• A human zygote's nucleus has 46 chromosomes
• Chromosomes consist of thousands of genes, which are the basic units of heredity, that construct a single protein.
• Chromosomes come in matching pairs, with each member similar in size, shape, and function.
• One chromosome of each pair is from the mother, the other is from the father.
• Each parent contributes 23 chromosomes to their offspring.
• Genes function as pairs on each chromosome, located at the same sites on their corresponding chromosomes.
• Genes are stretches of DNA, a "double-helix" molecule resembling a twisted ladder that provides the chemical basis for development.
• DNA can uniquely duplicate itself by splitting its ladder-like molecule and replicating the missing parts.
• DNA replication capability enables a single-celled zygote to develop into a complex human being.

Growth of the Zygote and Production of Body Cells

• The zygote replicates through mitosis as it moves through the fallopian tube towards the uterus.
• The zygote divides into two cells, then four, then eight, and so on.
• The cell duplicates its 46 chromosomes before each division.
• Duplicate sets move in opposite directions, and the cell divides, resulting in two new cells.
• Each new cell has 23 pairs of identical chromosomes (46 total) and the same genetic material as the original.

Germ Cells

• Germ cells produce gametes (sperm in males and ova in females).
• Germ cell reproduction differs from mitosis but shares some characteristics.
• Germ cells join with gametes to form a unique individual.
• Germ cells reproduce through meiosis.
• Male germ cells in the testes and female germ cells in the ovaries produce sperm and ova.
• Meiosis starts by duplicating the germ cell's 46 chromosomes.
• Crossing-over often occurs, where adjacent duplicated chromosomes exchange genetic material.
• This creates new hereditary combinations.
• Pairs of duplicated chromosomes segregate into two new cells, each containing 46 chromosomes.
• The new cells divide again, forming gametes with 23 single chromosomes.
• At conception, a sperm and ovum unite with 23 chromosomes each, producing a zygote with 46 chromosomes.

Hereditary Uniqueness

• During meiosis, chromosome segregation is random.
• Chromosome pairs segregate independently, resulting in many different chromosome combinations.
• Each parent can produce over 8 million different genetic combinations due to 23 chromosome pairs.
• A couple could theoretically conceive 64 trillion babies without producing two children with the same genes.
• The odds of exact genetic replication in siblings is less than 1 in 64 trillion.
• This is because the crossing-over process alters the genetic composition of chromosomes, increasing possible variations.

Multiple Births

• Monozygotic (identical) twins share a genotype.
• Monozygotic twins come from a single zygote that splits into identical cells.
• Identical twins occur in ~1 out of 250 births.
• Monozygotic twins' developmental progress should be similar if genes matter.
• Dizygotic (fraternal) twins are more common
• Dizygotic twins occur in ~1 out of 125 births
• Dizygotic twins come from a mother releasing two ova at once.
• Fraternal twins have no more genes in common than any pair of siblings.

Male or Female?

• Sex differences have a hereditary basis seen when examining chromosomes of men and women.
• Karyotypes reveal that 22 of the 23 pairs of human chromosomes (autosomes) are similar in males and females.
• Sex is determined by the 23rd pair (sex chromosomes).
• The 23rd pair in males consist of an X chromosome and a Y chromosome.
• The 23rd pair In females are both X chromosomes.
• Fathers determine the sex of their children

Genetic Makeup

• Sperm contains either an X or Y chromosome.
• Ova contain an X chromosome.
• Sex is determined by whether an X or Y sperm fertilizes the ovum.
• A genetically unique individual inherits thousands of genes on 46 chromosomes from their parents.

What Do Genes Do?

• Genes are needed to create amino acids
• Amino acids form enzymes and other proteins, which are necessary for new cells.
• Genes regulate melanin production, affecting eye color.
• More melanin corresponds to brown eyes, less pigmentation corresponds to blue or green eyes.
• Genes guide cell differentiation, helping make parts of the brain, central nervous system, circulatory system, bones, and skin.
• Genes influence and are influenced by the biochemical environment during development.
• This means a cell might become part of an eyeball or an elbow based on surrounding cells.

Hereditary Disorders

• ~5 out of 100 infants have congenital problems
• Congenital defects are present at birth and may not be detectable immediately.
• Huntington's disease is present from conception.

Chromosomal Abnormalities

• Germ cell division can distribute chromosomes unevenly during meiosis.
• Some gametes may have too many or too few chromosomes.
• Chromosomal abnormalities are lethal, resulting in failure to develop or spontaneous abortion.
• About 1 in 250 children are born with too many or too few chromosomes.

Abnormalities of the Sex Chromosomes

• Chromosomal abnormalities can affect the 23rd pair (sex chromosomes).
• Males may have an extra X or Y chromosome (XXY or XYY).
• Females may inherit a single X chromosome (XO) or three (XXX), four (XXXX), or five (XXXXX) X chromosomes.
• Each condition affects development, appearance, fertility, and intellectual capacity differently.

Abnormalities of the Autosomes

• Autosomal abnormalities affect the 22 pairs of chromosomes.
• The most common autosomal abnormality occurs when an abnormal sperm or ovum with an extra autosome combines with a normal gamete.
• The result is a zygote with 47 chromosomes (2 sex chromosomes and 45 autosomes).
• The extra chromosome forms a trisomy with one of the 22 pairs of autosomes.
• The most frequent autosomal abnormality is Down syndrome (trisomy-21)
• Down syndrome occurs in 1 out of 800 births.
• Children with Down syndrome are mentally retarded (average IQ of 55).
• Down syndrome may include congenital eye, ear, and heart defects.
• Children with Down syndrome often have a sloping forehead, protruding tongue, short limbs, a flattened nose, and almond-shaped eyes.
• Children reach developmental milestones at a slower pace.
• Parental support and stimulation contribute to developmental progress.

Genetic Abnormalities

• Healthy parents may be surprised to learn their child has a hereditary defect.
• Most genetic problems are recessive traits that need both parents to carry and pass on the harmful allele.
• Sex-linked defects display in male children who inherit recessive alleles on their X chromosome from their mother.
• Genetic abnormalities can be detected before birth.
• They affect major organ systems and although rare, some form of treatment can mitigate the harm.
• Dominant alleles cause some genetic abnormalities, which display in children who inherit the allele from either parent.
• Genetic abnormalities can result from mutations.
• Mutations are changes in the chemical structure of genes that produce a new phenotype.
• Mutations can occur spontaneously or be induced by toxins, radiation, or chemicals.

Predicting, Detecting, and Treating Hereditary Disorders

• Genetic counseling predicts the likelihood of hereditary defects.
• "Genetic counseling" covers both chromosomal and genetic abnormalities.
• Counselors are trained in genetics, family histories, and counseling.
• Counselors work as geneticists, medical researchers, or practitioners.
• Genetic counseling is helpful for couples with family histories of hereditary disorders to determine the risks.
• Counselors get a full family history (pedigree) from each prospective parent.
• Pedigrees estimate the risk of chromosomal or genetic disorders.
• Pedigrees determine the likelihood of certain disorders like diabetes or muscular dystrophy.
• DNA analyses from blood tests determine if parents carry genes for Huntington's disease and fragile-X syndrome.

Detecting Hereditary Disorders

• Amniocentesis is a prenatal screening recommended for older pregnant women (over 35).
• It involves inserting a needle to withdraw amniotic fluid of the fetus.
• Fetal cells in the fluid are tested to determine sex and for chromosomal abnormalities (Down syndrome) and for genetic disorders like Tay-Sachs
• amniocentesis can trigger miscarriage.
• Miscarriage risks for women under 35 are thought to be less than the risk of the birth defect itself.
• Amniocentesis is not performed before the 11th-14th week of pregnancy
• Results will not come back for two weeks, so parents have little time to decide if abortion is right for them.

Treating Hereditary Disorders

• Prenatal detection of defects can create a moral problem for some couples.
• Couples must decide to either terminate the pregnancy or have a baby who declines and dies young.
• Medical science can now treat disorders like phenylketonuria (PKU)
• PKU is a metabolic disorder that affects the nervous system.
• If untreated, those with this disease become severely hyperactive and retarded over time.

Hereditary Influences on Behavior

• Behavioral geneticists study genotypes (inherited genes) and phenotypes (observable traits/behaviors).
• Behavioral geneticists are NOT strict hereditarians and recognize that the environment also affects the outcome.
• Environmental factors like diet can affect physical characteristics AND long-term effects of genotype on behavioral characteristics.
• Behavioral geneticists understand that strong hereditary components can be modified by environmental influence.

Behavioral Genetics vs Ethologists

• Behavioral geneticists vary from ethologists and study the bases of development.
• Ethologists study similar inherited attributes that characterize species and contribute to common development.
• Behavioral geneticists focus on the biological bases for variations among members of the species.
• They are concerned with determining how genetic variation affects behavior.

Methods of Studying Hereditary Influences

• Genes can be studied in animals through selective and controlled breeding
• Selective Breeding manipulates the genetic makeup of animals to study hereditary influences on certain behaviors.
• Tryon's (1940) experiment selectively bred rats for maze-learning ability.
• Rats were separated into bright (few errors) and dull (many errors) learners.
• Tryon mated bright rats only with bright rats, and dull rats only with dull rats.
• Environments of the rats were matched in an attempt to control for environmental factors.

Family Studies

• Family studies are used in humans
• Family Studies are less invasive
• People who live together are compared on attributes.
• If the attribute is heritable, similarity between individuals living in the same areas should increase as a function of kinship.
• Kinship is an estimation (fraternal twins share ~50% of the same genes).

Twin Studies

• Twin studies are a type of family study.
• Twin studies ask whether identical twins reared together are more similar than fraternal twins.
• Gene presence suggests identical twins should be more similar.
• Twins share 100% of genes and fraternal twins share 50% of genes.

Estimating the Contribution of Genes and Environment

• Traits that determine whether a trait is genetically influenced and to what degree environment accounts for individual difference can use mathematical calculations.
• Concordance rates study whether people display a trait or not.
• Concordance rates calculated by percentages in pairs of people, such as parents and twins.

Environmental Influences

• Nonshared environmental influences (NSE) are experiences unique to the individual that cause family members to be different.

• Identical twins raised together are not perfectly similar in IQ as their correlation is +.86.

• Shared environmental influences (SE) are experiences individuals share that make them more similar.

• Children growing up in the same home tend to be more similar.

Myths about Heritability Estimates

• Heritability coefficients estimate gene contribution to human attributes.
• These are often misunderstood.
• People believe heritability coefficients can tell them whether they inherited a trait. This is not the case.
• Heritability refers to how differences among individuals are related to inherited genes.