HPS111 Lecture Notes - Lecture 3: Phenotypic Trait, Twin Study, Heredity
Chapter 3
Heritability – the extent to which genetic individual differences contribute to individual differences in observed
behaviour (or phenotypic individual differences).
Genotype – the specific genetic make-up of an individual. It is present from conception in the womb.
Phenotype – An individual’s observable characteristics. It can be affected by other genes as well as by the
environment.
Chromosome – found in the nuclei, the material of heredity, a double-stranded and tightly coiled molecule of
DNA. Every cell in the body carries 23 pairs of chromosomes (43 chromosomes), each containing numerous
genes that regulate every aspect of cellular functioning.
The sex cell contains 23 chromosomes. At conception, the 23 chromosomes from the egg combine with the 23
chromosomes from the sperm to form a new cell called a ‘zygote,’ containing 46 chromosomes.
All the information of heredity is encoded in combinations of four chemical bases – adenine, guanine, cytosine,
and thymine.
Genes – biological units of heredity. The average gene has 3000 ATGC base pairs. Each gene carries the ATGC
codes for manufacturing specific proteins, as well as when and where in the body they will be made.
Allele – alternative forms of a gene that produce different characteristics. i.e. there is an allele that produces blue
eyes and a different one that produces brown eyes.
Dominant vs Recessive gene:
If a gene in the pair received from the mother and father is dominant, the particular
characteristic that it controls will be displayed. E.g. in humans, brown eyes is a dominant gene. If a child
inherits a dominant gene for brown eyes and recessive gene for blue eyes, he will have brown eyes. However,
the blue-eyed trait will remain hidden in his genotype and may be passed on to his offspring.
If the gene is recessive, the characteristic will not show up unless the partner gene inherited from the other
parent is also recessive. E.g. a person will only have blue eyes, a recessive gene, if both his parents contributed
recessive genes for blue eyes.
Polygenic transmission – where, a number of genes pairs combine their influences to create a single
phenotypic trait.
The human genome consists of 25000 genes.
Behaviour genetics – studies how heredity, and environmental and social factors influence
psychological characteristics. i.e. they try to determine the relative influence of genetic,
environmental and social factors in accounting for individual differences in behaviour.
The probability of sharing any particular gene with your parents is 0.5 or 50%. If you have
siblings, then you have a 0.5 or 50% chance of sharing the same gene with them. You have a 0.25 or 25%
chance of sharing any particular gene with your grandparents. If you are an identical twin then u have a 1.00 or
100% chance of sharing any particular gene with your twin.
Research methods used to estimate the influence of genetic contribution:
Family studies – researchers study relatives to determine if genetic similarity is
related to similarity on a particular trait.
Adoption studies – researchers study people who were adopted early in life and
compare some of their characteristics with those of their biological parents, with whom they share a genetic
endowment, as well as compare it with their adoptive parents, with who they share no genes but a common
environment.
Twin studies – compares trait similarities in identical (monozygotic) and fraternal (dizygotic) twins.
Monozygotic twins develop from the same fertilised and egg and therefore genetically identical. Dizygotic twins
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Document Summary
Heritability the extent to which genetic individual differences contribute to individual differences in observed behaviour (or phenotypic individual differences). Genotype the specific genetic make-up of an individual. It is present from conception in the womb. It can be affected by other genes as well as by the environment. Chromosome found in the nuclei, the material of heredity, a double-stranded and tightly coiled molecule of. Every cell in the body carries 23 pairs of chromosomes (43 chromosomes), each containing numerous genes that regulate every aspect of cellular functioning. At conception, the 23 chromosomes from the egg combine with the 23 chromosomes from the sperm to form a new cell called a zygote," containing 46 chromosomes. All the information of heredity is encoded in combinations of four chemical bases adenine, guanine, cytosine, and thymine. The average gene has 3000 atgc base pairs.
