Biological Foundations of Development

Chapter 3

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Genetics• The study of how genes and traits are passed from

parents to children– Eye color– Hair color– Height– Body build– Blood types– Intelligence– Gender

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Genetics• Genome

– The complete set of genes– “Book of instructions” for making a body

• Genes– Provide information about how proteins, the

building blocks of the body, are to be constructed

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Cell StructureCytoplasm: Substance of a cell outside of the nucleus

Nucleus: Central point of cell, contains genetic coding for maintaining life systems and issuing commands for growth and reproduction

Chromosomes: 46 in each Nucleus (23 pairs)

Genes: bands on chromosomes (thousands of genes)

DNA on genes (billions of DNA)

Cell: Basic unit of all living mater (Adults= over 10 trillion cells)

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Chromosomes

• Tightly coiled ribbons of DNA• Genes are located on the “arms” or “legs”• Reproductive or germ cells have the same amount

– 46 chromosomes • 23 are from mother• 23 are from father

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Cell Division• Mitosis

– Cells divide by copying the DNA

– When cell division occurs, the DNA “unzips” and duplicates itself

– Each of the resulting cells contain 23 pairs of chromosomes

– 46 chromosomes total

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Cell Division• Meiosis

– Gametes, the ova and sperm, undergo one duplication of chromosomes

– each cell has a full set of genetic material

– Each cell then undergoes another division; creating ½ sets of chromosomes:

– Half of each parent’s genetic information

– 23 chromosomes total

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Baby’s Gender• Female Sex Cells XX – ovum – 23rd chromosome

• Male Sex Cells XY – sperm - 23rd chromosome

• Conception is the union of ovum and sperm (gametes)

• Gender determined by the father

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Multiple Births• Monozygotic (Identical Twins)

– 1 egg + 1 sperm– Cell divides– Always the same gender

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Multiple Births• Dizygotic (Fraternal Twins)

– 2 Eggs + different sperm– Will look different, can be different gender

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Multiple Births• Conjoined Twins

– Ovum splits apart, separation not complete– Babies joined at some part of their bodies

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Multiple Births• Four Factors that may contribute…

1. History in the family2. Increased hormones naturally3. Fertility drugs4. Age 32 - 36

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Transmission• Dominant

– A gene’s code that is expressed when only one copy of the gene is present

• Recessive– A gene’s code that is not

expressed in the presence of a dominant gene

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Sex-Linked Transmission

• X chromosome carries a great deal of information

• Y chromosome carries very little information

• remember…– females: XX– males: XY

• Recessive transmission of genetic diseases on the sex chromosomes is more common

• Females tend to be carriers, while males tend to be those affected

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Polygenic Transmission

• The combined effect of multiple genes to produce an outcome– e.g. height, skin color, intelligence

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Chromosomal Abnormalities

• Down Syndrome

• Sex Chromosome Abnormalities– Turner Syndrome– Trisomy-X Syndrome– Klinefelter’s Syndrome– XYY Syndrome

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Down Syndrome• Trisomy 21

• Characteristics– Specific facial appearance– Heart and eye defects– Visual or hearing

impairments– Low muscle tone– Mental retardation

• Higher risk for mothers over 35 years old

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Turner Syndrome

• Cause – Missing X chromosome – Missing part of the X chromosome

• Characteristics– No ovaries/infertile– Short– Broad chests– Webbed neck– Normal verbal skills– Poor spatial skills– Heart, kidney and thyroid problems

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Trisomy-X Syndrome

• Females with extra X chromosome (XXX)

• Characteristics– Appearance normal– Lower intelligence– Quiet, passive– May have delayed speech – May have delayed motor skills

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Klinefelter’s Syndrome

• Males born with an extra X chromosome (XXY)

• Characteristics– Sterile/immature testes– Long limbs– Abnormal pubertal development– May have

• Mental retardation• Language deficits• Attention problems

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XYY (Supermale) Syndrome

• One or more extra Y chromosomes

• Attributed to father

• Characteristics– Appear normal– Severe acne during adolescence– May exhibit

• Lower intelligence• High impulsivity

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Prenatal Genetic DiagnosesThere are some genetic tests that can help determine if there is an issue with the developing fetus:

•Ultrasound

•Chorionic Villus Sampling

•Alpha Fetoprotein Screen

•Amniocentesis

•Genetic Counseling

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Ultrasound• Sound wave echoes are

used to form a live video image of a fetus

• Can be used for diagnosis of birth defects

• Very safe

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Chorionic Villus Sampling (CVS)

• Conducted at 9-12 weeks gestation

• Catheter is inserted

• Fetal cells are removed and tested for various genetic disorders

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Alpha Fetoprotein (AFP) Detection• Conducted at 16-18 weeks gestation

• Mother’s blood is assayed for AFP levels– high levels indicate possible neural tube defects– low levels indicate possible chromosomal abnormalities

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Amniocentesis• Conducted at 15-19 weeks

gestation

• Needle is inserted into abdomen

• Fetal skin cells are removed and assayed for genetic abnormalities

• Low risk

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Genetic Counseling• Birth defects

– Occur in 1/33 births– Are the leading cause of infant death

• Genetic counselors – Help determine risk– Provide counseling for families with an infant born with

genetic problems

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Genetic Counseling• Recommended for

– Parents of children with genetic disorders– Parents with a known genetic disorder– Women who have repeatedly miscarried– Older pregnant women– Couples of certain ancestry/ethnicities– Couples who are related to each other

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Conception and Infertility

• Ovulation– For pregnancy to occur,

a human egg, or ovum, and a sperm cell meet and share their genetic material

– Each month a woman releases at least one ovum from her ovaries during ovulation

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Phases of Ovulation• Menstrual phase

– Menstruation occurs because the lining of the uterus degenerates and is expelled in a bloody discharge

• Follicular phase– The lining of the uterus grows back and thickens, influenced

by the female hormone estrogen. Mostly likely time pregnancy occurs

• Luteal phase– Different hormones are secreted (i.e., progesterone) that

prepare the uterus for the arrival of a fertilized ovum

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Phases of Ovulation

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Spermatogenesis

• Process of sperm development

• Spermatozoa– Mature sperm cells– Contain either an X or Y

chromosome– Form in 72 days

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Sperm• Produced at a rate of 100 per second

• 200-500 million per ejaculation

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Fertilization• Ova must be fertilized in 24-48 hours after

release

• Sperm– Millions are released– Viable for 3 days– About 100-1,000 reach the egg– Fertilization occurs when a single sperm

penetrates the outer layer of the ovum

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Infertility• 8.5% of couples have fertility issues

• Defined as – No conception after 12 months of unprotected sex– Being unable to carry a fetus to term

• Causes– 50% maternal origin– 20% paternal origin– 30% of unclear origin

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Intrauterine Insemination (IUI)

• Sperm directly injected into the uterus when the woman is ovulating

• 8,000 children a year in the U.S. are conceived through IUI

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In Vitro Fertilization• Ova

– Removed from woman’s body– Fertilized with sperm in a petri

dish

• Zygotes are transferred to the uterus

• Common method

• Can result in multiple births if multiple eggs are transferred

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Embryo Transfer

• Used when both partners are infertile

• Donors provide egg and sperm

• Fertilized eggs are transplanted into woman

• Rare

• Low success rate

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Surrogate Mothers

• Woman is artificially inseminated with sperm

• Carries fertilized egg through pregnancy and birth

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Infertility• 14% of births use artificial reproductive technologies

• Rates of multiples has increased

• Time consuming, expensive, emotionally draining

• Most resulting children are healthy

• Ethical questions