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Chapter Two

Heredity and Conception

Fall 2017

Rathus

2.1 The Influence of Heredity on Development

The Nature of Nature

The Influence of Heredity on Development

Heredity

Based on biological transmission of traits and characteristics from one generation to another

Genetics

Branch of biology that studies heredity

Genetic (inherited) Influences

Physical traits

Intelligence, emotional, and personality traits

Behavioral traits and psychological issues

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About 120 – 150 boys are conceived for every 100 girls. TRUE. It is true that 120–150 boys are conceived for every 100 girls. However, male fetuses suffer a higher rate of spontaneous abortion (miscarriage) than females, often during the first month of pregnancy. At birth, boys outnumber girls by a ratio of only 106 to 100. Boys also have a higher incidence of infant mortality, which further equalizes the numbers of girls and boys.

Sperm travel about at random inside the woman’s reproductive tract, so which one reaches the ovum first is a matter of luck. FALSE. It is not true that sperm travel about at random inside the woman’s reproductive tract, so that reaching the ovum is a matter of luck. Sperm cells are apparently attracted by the odor of a chemical secreted by ova. They seem to be “egged on” (pardon the pun) by a change in calcium ions that occurs when an ovum is released (Olson et al., 2011).

What Are Chromosomes and Genes?

Chromosomes

Found in cells

23 pairs of rod-shaped structures

Genes

Segments within chromosomes

Regulate development of traits

Transmitted by single gene or may be polygenic (determined by combinations of pairs)

20,000 to 25,000 genes in every cell

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Extensive athletic activity may contribute to infertility in the male. TRUE. Extensive athletic activity can contribute to infertility in the male. Sometimes the sperm count is adequate, but other factors (such as prostate or hormonal problems) can deform sperm or deprive them of their motility (their ability to move actively). Motility can also be impaired by the scar tissue from infections, such as STIs.

“Test-tube” babies are grown in a laboratory dish throughout their 9-month gestation period. FALSE. It is not true that test-tube babies are grown in a test tube—but close. In this method, more technically known as in vitro fertilization (IVF) , ripened ova are removed surgically from the mother and placed in a laboratory dish. The father’s sperm are also placed in the dish. One or more ova are fertilized and then injected into the mother’s uterus to become implanted.

You can select the sex of your child. TRUE. Preimplantation Genetic Diagnosis (PGD) is a fool-proof sex-selection method, but it is medically invasive and expensive, and successful implantation cannot be guaranteed.

What Are Chromosomes and Genes?

DNA

Large strands make up genes

Double spiral (helix)

Composed of phosphate and simple sugar

Base pairs adenine with thymine (A – T) or cytosine with guanine (C – G)

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Double Helix of DNA

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Figure 2.1 Double Helix of DNA. DNA consists of phosphate, sugar, and a number of bases. It takes the form of a double spiral, or helix.

What Are Mitosis and Meiosis?

Mitosis

Cell division by which growth occurs and tissues are replaced

Strands of DNA break apart, duplicate, and are rebuilt

Result is identical copies of DNA strand

Mutations are the exception

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Want to learn more about the Human Genome Project? Click HERE

Mitosis

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Figure 2.2 Mitosis. (a) A segment of a strand of DNA before mitosis. (b) During mitosis, chromosomal strands of DNA “unzip.” (c) The double helix is rebuilt in the cell as each incomplete “rung” combines with appropriate molecules. The resulting identical copies of the DNA strand move apart when the cell divides, each joining one of the new cells.

What Are Mitosis and Meiosis?

Meiosis

Cell division by which sperm and ova are produced

23 chromosome pairs divide

Result is a new cell with only 23 chromosomes

22 pairs are autosomes

23rd pair are sex chromosomes

Determines sex: X from mother and X or Y from father

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The 23 Pairs of Human Chromosomes

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read more about genetic mutations on the Genetics Home Reference website

What are Identical and Fraternal Twins?

Monozygotic (Identical) Twins (MZ)

Derived from a single zygote that has split in two

Dizygotic (Fraternal) Twins (DZ)

Derived from two zygotes

Share 50% of genetic material

Probability of twins increases

Maternal age – less regular ovulation

Use of fertility drugs

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What are Dominant and Recessive Traits?

Traits are determined by pairs of genes

Each member of pair is an allele

Homozygous

Both alleles for a trait are the same

Heterozygous

Alleles for a trait are different

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Truth or Fiction Revisited

Your father determined whether you are female or male. (TRUE) It is true that your father determined whether you are a female or a male. The mother’s ova have only X chromosomes. If the father’s sperm cell supplies another X sex chromosome, the baby will be female. If the father’s sperm cell supplies a Y sex chromosome (named after its Y shape), the baby will be male.

What are Dominant and Recessive Traits?

Gregor Mendel

Established the laws of heredity

Averaging

Effects of both alleles are shown

Incomplete dominance or codominance

Law of Dominance

Dominant allele paired with recessive allele

Dominant allele appears in offspring

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Transmission of Dominant and Recessive Traits

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Figure 2.4 Transmission of Dominant and Recessive Traits. Each of these two brown-eyed parents carries a gene for blue eyes. Their children have an equal chance of receiving a gene for brown eyes or a gene for blue eyes from each parent.

Dominant and Recessive Traits

Combinations of dominant and recessive genes

Carriers

Chromosomal or genetic abnormalities

Occur in autosomes or sex chromosomes

May be caused by a single gene or combinations

Multifactorial problems

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2.2 Chromosomal and Genetic Abnormalities

Priya and HD

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Truth or Fiction Revisited

Brown eyes are dominant over blue eyes. (TRUE) If one parent carries genes for only brown eyes and the other parent carries genes for only blue eyes, the children will have brown eyes. But brown-eyed parents can also carry recessive genes for blue eyes so that if both brown-eyed parents have recessive genes for blue eyes, their children have a one in four chance of having blue-eyes.

Chromosomal Abnormalities: Down Syndrome

Cause: extra chromosome on 21st pair

Probability increases with increased age of parent

Characteristics of children

Facial features

Deficits in cognitive, language, and motor development

Adjustment problems

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Down Syndrome

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Truth or Fiction Revisited

You can carry the genes for a deadly illness and not become sick yourself. (TRUE) It is true that you can carry the genes for a deadly illness and not become sick yourself. This occurs when genes for the illness are recessive, and dominant genes cancel their effects.

Figure 2.5. Down Syndrome. The development and adjustment of children with Down Syndrome are related to their acceptance by their families. Children with Down Syndrome who are reared at home develop more rapidly and achieve higher levels of functioning than those who are reared in institutions.

Sex-Linked Chromosomal Abnormalities

Most are infertile

Male with extra sex chromosome

XYY: extra Y chromosome

XXY:- Klinefelter syndrome

Female with abnormal number of sex chromosomes

X: Turner syndrome

XXX: Triple X syndrome

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Genetic Abnormalities

Phenylketonuria (PKU)

Huntington Disease

Sickle-Cell Anemia

Tay-Sachs disease

Cystic Fibrosis

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to learn more about recent advances in detection and treatment of Down syndrome CLICK HERE

Sex-Linked Genetic Abnormalities

Carried on X sex chromosome

Hemophilia

Duchenne muscular dystrophy

Diabetes

Color blindness

Some types of night blindness

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Determining Probability of Genetic or Chromosomal Abnormalities

Genetic Counseling

Addresses probability of genetic abnormalities

Information about couple’s genetic heritage

Prenatal testing

Amniocentesis

Chorionic villus sampling (CVS)

Ultrasound

Blood tests

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Genetic Counseling and Prenatal Testing

Amniocentesis

Usually performed on mother about 14-16 weeks after conception

Can detect more than 100 chromosomal and genetic abnormalities in fetus

Indicates the sex of the baby

Some risk of miscarriage

Improved ultrasound and blood tests is reducing use of amniocentesis

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Learn more about amniocentesis HERE

Amniocentesis

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Genetic Counseling and Prenatal Testing

Chorionic Villus Sampling (CVS)

Can diagnosis abnormalities earlier than amniocentesis

Equal or slightly higher risk of miscarriage, compared to amniocentesis

Ultrasound

Sonogram “picture” of fetus

Used to track fetus’ growth and to determine age, sex, and structural abnormalities

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Genetic Counseling and Prenatal Testing

Blood Tests

Can reveal presence of recessive genes in parents

Alpha-Fetoprotein (AFP) assay

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Sonogram

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Figure 2.7 Sonogram of a 5-Month-Old Fetus. In the ultrasound technique, sound waves are bounced off the fetus and provide a picture, called a sonogram, that enables professionals to detect various abnormalities

2.3 Heredity and the Environment

Nature versus Nurture

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Reaction Range

Range of possibilities for the expression of the trait

Genotypes

Sets of traits inherited from parents

Phenotypes

Actual sets of traits

Product of genetic and environmental influences

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In addition to inheritance, the development of our traits is influenced by nutrition, learning, exercise, and—unfortunately—accident and illness.

Canalization

Environmental influences on genotype within reaction range

Canalization = sequence of development is invariant

Infant motor development

Less canalization

Intelligence

Personality

Environmental restrictions, such as scarcity of food, may prevent children from reaching heights within their reaction ranges. However, if food becomes more readily available, there is a tendency to “snap back” into the genetically determined “canal”

Genetic – Environmental Correlation

Passive Correlation

Environment that child is placed into

Evocative Correlation

Child’s genotype elicits responses

Active Correlation

Environment child chooses

Passive:The correlation between the genetic endowment parents give their children and the environments in which they place their children.

Evocative: The correlation between the child’s genetic endowment and the responses the child elicits from other people.

Active: The correlation between the child’s genetic endowment and the choices the child makes about which environments they will seek.

Epigenetic Framework

Development reflects the continual bidirectional exchanges between genetics and environmental influences

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The Effects of Genetics and Environmental Influences

Kinship Studies

Genetic closeness of relatives

Twin Studies

Monozygotic twins share 100% of genes

Dizygotic twins share 50% of genes (same as other siblings)

Reared together versus reared apart

Adoption Studies

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2.4 Conception

Against All Odds

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Conception

Ovarian follicle ruptures releasing the egg

Hundreds of millions of sperm are ejaculated

Only a few thousand survive through the cervix and uterus

A few hundred bombard the ovum in the fallopian tube

Sperm and ovum unite

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Ova

Begin to mature at puberty

Monthly release of mature egg into fallopian tube

Egg is propelled by cilia and, perhaps, by contractions in the wall of the fallopian tube

If not fertilized, egg is discharged along with endometrium

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Female Reproductive Organs

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Figure 2.8 Female Reproductive Organs. Conception is something of an obstacle course. Sperm must survive the pull of gravity and vaginal acidity, risk winding up in the wrong fallopian tube, and surmount other hurdles before they reach the ovum.

Sperm Cells

Each contains 46 chromosomes, including one X and one Y

Self propelled and smaller than ova

Sperm with “Y” chromosome swim faster than sperm with “X” chromosome

More boys are conceived than girls

From 200 to 400 million in ejaculate; only 1 in 1,000 arrive in vicinity of ovum

Attracted by chemical odor secreted by ova

Conception

Figure 2.9 Human Sperm Swarming Around An Ovum in a Fallopian Tube. Fertilization normally occurs in a fallopian tube, not in the uterus. Thousands of sperm may wind up in the vicinity of an ovum, but only one fertilizes it. How this sperm cell is “selected” remains one of the mysteries of nature.

Sperm Cells

Sperm must penetrate gelatinous layer around ova

Sperm cells secrete an enzyme that briefly thins the layer

When one sperm penetrates, the layer thickens, locking other sperm out

Chromosomes from sperm cell combines with egg chromosomes to form 23 new pairs with unique genetic instructions

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2.5 Infertility and Assisted Reproductive Technology

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What Are the Causes of Infertility?

Fertility problems among men

Low sperm count

Deformed sperm

Low sperm motility

Diseases

Injury of testes

Autoimmune responses

Causes

Genetic factors, environmental poisons, diabetes, STI’s, overheating testes, pressure to testes, aging, and drugs

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Truth or Fiction Revisited

Girls are born with all the egg cells they will ever have. (TRUE) At birth, women have 300,000-400,000 ova in each ovary, although they will only ovulate some 400 of these during their lifetimes (Adhikari & Liu, 2013).

What Are the Causes of Infertility?

Major fertility problems among women

Irregular or absence of ovulation

Fertility drugs are used to cause women to ovulate

Declining hormone levels due to aging

Endometriosis

Obstructions or malfunctions of reproductive tract

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What Are the Causes of Infertility?

Causes

Hormone irregularities

Stress

Malnutrition

Infections that produce obstructions, as from scarring

Pelvic inflammatory disease (PID)

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Truth or Fiction Revisited

About 120 – 150 boys are conceived for every 100 girls. (True) It is true that 120–150 boys are conceived for every 100 girls. However, male fetuses suffer a higher rate of spontaneous abortion (miscarriage) than females, often during the first month of pregnancy. At birth, boys outnumber girls by a ratio of only 106 to 100. Boys also have a higher incidence of infant mortality, which further equalizes the numbers of girls and boys.

Sperm travel about at random inside the woman’s reproductive tract, so which one reaches the ovum first is a matter of luck. (False) It is not true that sperm travel about at random inside the woman’s reproductive tract, so that reaching the ovum is a matter of luck. Sperm cells are apparently attracted by the odor of a chemical secreted by ova. They seem to be “egged on” (pardon the pun) by a change in calcium ions that occurs when an ovum is released (Olson et al., 2011).

How Are Couples with Fertility Problems Assisted?

Artificial Insemination

Sperm injected into mother’s uterus

In Vitro Fertilization

Ova and sperm are fertilized, then implanted in mother’s uterus

Donor IVF

Ovum harvested from donor woman; fertilized in vitro and implanted in recipient’s uterus

Embryonic transplant

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How Are Couples with Fertility Problems Assisted?

Surrogate Mothers

“Substitute” who carries a baby to term for another woman

Ethical and legal risks because the surrogate mother may not want to give up the baby

Adoption

Greater diversity of adopted children and adoptive parents

Consideration of adopted children needs

Relinquishing mothers also experience effects