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PSY 302 Lecture Notes - Chromosome Abnormality, Allosome, Y Chromosome

Course Code
PSY 302
Alba A.

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Class Notes - Chapter 3 Biology and Behavior
I. Nature and Nurture
Both heredity and environment influence individuals’ characteristics.
Individuals differ from one another by only about 1 to1.5% of their genes.
Genetic and Environmental Influences
People are shaped by several interacting factors
See Figure 3.1 in Siegler et al., 2010
Genotype: the genetic material an individual inherits
Phenotype: the observable expression of the genotype, including body characteristics and
Environment: includes every aspect of the individual, and his or her surroundings, other
than genes
these 3 elements are involved in 4 relations that are fundamental in the development of
every child:
Four Fundamental Relations
1. Parents’ genetic contribution to the child’s genotype conan is a really tall
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
An example of the model: Conan is really tall & hair texture the way the genotype will
be expressed is influence by the environment
Relation 1: Parents’ and Child’s Genotypes
Genetic material is passed on by chromosomes
o Carry all the instructions involved in the formation and functioning of an
o Genes: sections of chromosomes; basic units of heredity for all living things; each
gene is a segment of DNA that is the code for the production of particular proteins
o Genes affect development and behavior only through the manufacture of proteins
Sex Determination
o Sex chromosomes determine an individual’s biological sex.
o Females have two X chromosomes in the 23rd pair, whereas males have X Y.
o X chromosomes have 3x more genes than Y. more genes which can possibly
that we can override the genes that are sex-linked. Ex. Colour blindness
o It is always the father who determines the sex of the offspring because half of the
male’s sperm contain an X chromosome and half contain a Y. when a Y bearing
sperm fertilize an egg, offspring will be male, etc
o A gene on the Y chromosome encodes the protein that triggers the formation of
the testes, which produce testosterone, which produces male body features.

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Why are children of the same two parents so genetically diverse?
- Several mechanisms contribute to genetic diversity among people:
Mutations: changes in sections of DNA caused by random or environmental factors;
those mutation that occur in germ cells can be passed on to offspring;
Crossing over: Sections of DNA switch from one chromosome to another during sex cell
production, further increasing genetic variability; as a result, some of the chromosomes
that parents pass on to their offspring are constituted differently from their own.
Random assortment: Chance determines which member of each pair of chromosomes
goes into the new sperm or egg; during germ-cell division, the 23 pair of chromosomes
are shuffled randomly, with chance determining which member of each pair goes into
each new egg or sperm; for each germ cell there are 2^23 or 8.4 million possible
combinations of chromosomes
Relation 2: Child’s Genotype and Phenotype
Although every cell in the body contains copies of all the genes received from parents,
not all are expressed
Only some of those a person’s genes are expressed at any one time in any given cell.
Several reasons:
Gene Expression
About a third of human genes have two or more different forms, known as alleles.
The alleles of a given gene influence the same trait or characteristic(eye color), but they
contribute to different developmental outcomes(brown, blue, hazel, gray eyes)
The dominant allele is the form of the gene that is expressed if present.
The recessive allele is not expressed if a dominant allele is present.
A person who inherits two of the same alleles for a trait is described as homozygous.
A person who inherits two different alleles for a trait is described as heterozygous.
Mendelian Inheritance Patterns
Fig 3.3
Sex linked inheritance:
- Suppose a woman inherits a recessive allele on the X chromosome from
her mother, chances are she will have a dominant allele on the X
chromosome from her father to suppress it.
- Suppose a man inherits the same recessive allele on the X chromosome
from his mother, chances are, because of the much smaller size of the Y
chromosome he inherited from his father, he will not have a dominant
allele to override it, so he will develop the trait
Polygenic Inheritance: When traits (phenotypic outcome) are governed by more than one gene
Applies to most traits and behaviours of interest to behavioral scientists
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