Biology and Behavior - CMUrakison/POCDclass5.pdf · Class 4: genetic influences Fundamental...

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Class 4: genetic inf luences

Biology and Behavior

How Children Develop (3rd ed.) Siegler, DeLoache & Eisenberg

Chapter 3

Biological Foundations of Development: Genetic Influences

How similar in appearance are you and your sibs to your parents? Some of us look like one parent, some both, some neither.

• GENOTYPE: Set of genetic traits a person inherits

• PHENOTYPE: Set of traits a person actually displays. Results from genotype and life experiences

Fundamental Relations

1. Parents’ genetic contribution to the child’s genotype

2. Contributions of the child’s genotype to his or her own phenotype

3. Contribution of the child’s environment to his or her own phenotype

4. Influence of the child’s phenotype on his or her environment

Parents’

Genotype

Child’s

Genotype

Child’s

Environment

Child’s

Phenotype

Biological Foundations of Development: Genetic Influences

• Watson and Crick (1953) introduced their model of the

structure of DNA

• Scientists are attempting to figure out the function of the 25,0000 genes that make up the human genome

Genetic Foundations

Mendel: Able to predict color of pea plant offspring.

Proposed genes: factors controlling physical traits

Genetic Code

• We all have trillions of cells.

• Each cell has a nucleus; and each nucleus has chromosomes.

• Chromosomes (46 in humans) carry genetic information

• They are paired…except for 23rd in male (XY).

• Chromosomes made up of deoxyriboneucleic acid – DNA. Looks like a twisted ladder – a gene is segment of this ladder.

• Genes instruct production of protein

– crucial for chemical reactions that trigger development

DNA can duplicate itself – mitosis. This allows a single celled ovum to become a many-celled adult

The Sex Cells

Individuals are created when two gametes (sex cells) – sperm/ovum - combine.

• Gametes have 23 chromosomes, which divide by meiosis. Meiosis involves gene shuffling – crossing over – where genes

exchange chromosomes.

• Meiosis leads to variability in offspring.

Why useful?

• No variability occurs when a duplicating zygote splits into two.

These are identical, or monozygotic, twins (4/1000). • Influencing factors:

• When two ova are released and both are fertilized – fraternal, or

dizygotic, twins. Influencing factors: Ethnicity (8/1000 whites, 14/1000 blacks), Age (rises to 35), less for women on poor diet.

Multiple Offspring

Boy or Girl?

• Every pair of chromosomes can be distinguished.

22 match (autosomes): Longest = 1, shortest = 22.

The 23rd consists of sex chromosomes

M=X Y F=X X

• Does an X or Y bearing sperm fertilize the ovum?

A gene on the Y chromosome encodes the protein that triggers the formation of the testes, which subsequently produce testosterone, which in turn molds maleness.

Patterns of Genetic Inheritance

Two+ genes (alleles) occur at same place of chromosome: one from Mom and one from Dad. Alike alleles: child displays inherited trait (child is homozygous). Different alleles, relationships between alleles determine trait (heterozygous).

Dominant-Recessive (D-R) Relationships The allele that affects characteristic is The allele that does not is Dominant alleles lead to certain dominant characteristics. Two recessive alleles lead to a recessive characteristic.

Dominant Recessive

Dark hair

Curly hair

Farsightedness

Type A blood

Double jointedness

Disorders and defects are often product of recessive alleles.

Polygenic Inheritance

When traits are governed by more than one gene

Applies to most traits and behaviors of interest to behavioral scientists

Genetic Transmission of Diseases and Disorders: cancer, heart disease, asthma, psychiatric disorders, behavior disorders

Phenylketonuria (PKU) is a common defect (1 in 8000) If both parents have recessive alleles = 1 in 4 children. PKU is lack of enzymes that neutralizes phenylalanine (proteins) – found in milk, eggs, fish. Causes lethargy by 5 months, retardedness by 1 year. But, can be treated:

•Development is slowed but deficit disappears by school years (Mazzocco et al., 1994).

Sickle cell anemia Affects only Black Americans. Two recessive alleles cause round blood cells to be sickle shaped. Cells clog blood vessels: causes pain, swelling. • Death is common before 20, few live after 40. Also caused by oxygen deprivation, recessive allele asserts itself. Why Black Americans?

Mutation

Where do harmful genes come from? Mutation: a sudden permanent change in a segment of DNA. May affect 1 or 2 genes, or many.

Some occur by chance, some from environmental agents. Example: Ionizing radiation before birth leads to miscarriage/children with defects.

X-Linked Inheritance

Males and females just as likely to inherit recessive disorders. BUT, when harmful allele is carried in the X chromosome, X-linked

inheritance occurs. • Females’ dominant X suppresses recessive X.

• Males more likely affected: their sex chromosomes do not match

– Y is shorter and often cannot override the X.

Normal

Father

Carrier Mother

Normal

Daughter

Carrier

Daughter

Normal

Hemophilic

Inheritance of Hemophilia, a Sex-Linked Disorder

From Seifert/Hoffnung, Child and

Adolescent Development, 5/e, Figure

3.8, p. 70. Used by permission of

Houghton Mifflin Company.

Examples of X-linked inheritance Red-Green Color Blindness : Affects males twice as often as females (Cohen, 1984) Miscarriage: infant and childhood death greater for males. Learning disabilities: behavior disorders: more common in boys. BUT: because Y-bearing sperm are lighter and quicker they reach ovum more quickly. Thus 120/100 boys to girl (and more boys conceived).

Genetic Imprinting

Over 1,000 traits follow D-R inheritance. But: Genetic imprinting: • alleles chemically marked so that the mother’s or father’s pair is

activated. For example: 1. Diabetes comes from father 2. Asthma and hay fever come from mother

Chromosomal Abnormalities

In addition to recessive alleles, chromosomal abnormalities cause serious developmental problems.

Often a chromosome pair does not separate properly. More serious than problem from single gene – involves many.

Mostly from 21st pair of chromosomes fail to separate. Individual has 3 of this chromosome rather than 2. • Children have distinct physical features:

1. Short, stocky, flattened face, almond shape eyes. 2. Often have heart defects, eye cataracts. Death: 14% by 1 year, 21% by 10, others by middle adulthood.

Down Syndrome (1 in 800; 1/1900 at 20; 1/130 at 39)

MATERNAL AGE (YEARS) 15 20 25 30 35 40 45 50

Figure 3.8

Relationship Between Maternal Age and the Incidence of Down Syndrome

Behavioral: 1. mental retardation, 2. speech problems, 3. slow motor development – All become worse with age. Smile less readily, no eye contact, explore less. BUT: Early intervention helps social and emotional skills – not

cognitive. So, it is genetic but environmental factors affect it.

The Family Study

The mainstay of modern behavioral genetics research

A trait of interest is measured among groups of

people who vary in genetic relatedness

Correlations for traits

measured for individuals who

Are genetically more

similar

Share the same

environment

Types of Family Studies

Twin-Study Designs:

Correlations for pairs of

monozygotic twins on a

trait of interest are

compared to those of

dizygotic twins

Adoption Studies:

Researchers examine

whether adopted children

are more like their

biological or their adopted

relatives

Minnesota Study of

Twins Reared Apart

Located and studied twin siblings who have not

met since they were infants

The team of investigators were struck by the

similarities they found in traits like IQ,

reaction to stress, and traditionalism

These similarities may be influenced by

selective placement and similarities in

fostering environments as well as by genetic

factors

Genetic Counseling Helps assess chance of hereditary disorders. High risk: 1. previous miscarriages, 2. genetic problems in family. There is an interview, family tree is established – pedigree. Blood tests, genetic analyses allow prediction of harmful carrier. Should parents “take a chance”? Or adopt?

Reproductive Choices

Methods of detection are available for those who are high risk.

Ultrasound: high-frequency sound waves give picture of fetus – gross physical defect can be seen.

Amniocentesis; most common: needle inserted to abdominal wall.

Uterus fluid is obtained. Cells examined.

• But, can cause miscarriages or injury to fetus.

Prenatal Diagnosis and Fetal Medicine

Biology and Behavior - CMUrakison/POCDclass5.pdf · Class 4: genetic influences Fundamental...

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