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Summary Block 5 Problem 1
Summary of problem 1 genetics and pregnancy (Shaffer & Kipp and Berk)
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Problem 1Shaffer & Kipp - Chapter 2 / Berk – Chapter 3
Each cell contains 46 chromosomes: a threadlike structure that store and transmit genetic information,
which are made of genes. Each chromosome contains genes: hereditary blueprints for development that
are transmitted unchanged from generation to generation. Chromosomes come in matching pairs. Each
parent contributes 23 chromosomes to each of their children.
Chromosomes are made up of deoxyribonucleic acid (DNA), a complex double helix molecule that
provides the chemical basis for development. They consist of pairs of chemical substances called bases. A
gene is a segment of DNA along the length of the chromosome.
DNA can duplicate itself through mitosis. Just before each division, the cell duplicates its 46 chromosomes
and the same genetic material as the original cell. Mitosis makes up muscles, bones, organs etc. Mitosis
continues throughout life., generating new cells that enable growth and replacing old ones that are
damaged.
Sex cells: Other than body cell, we have germ cells that serve to produce gametes. This is a different type
of cell reproductions called meiosis: the process by which a germ cell divides, producing gametes (ova and
sperm) that each contain half of the parent cells original complement chromosome.
The germ cell first duplicates it’s 46 chromosomes. Then crossing over takes place and adjacent duplicated
chromosomes cross and break at one or more points along their length to exchange segments of genetic
material. This creates new and unique hereditary combinations. Then pairs of duplicated chromosomes
divide into two cells, each contains 46 chromosomes. Finally, these cells divide and the resulting gametes
contain 23 single chromosomes. In males, 4 sperms are produced and this continues throughout his life. In
females only 1 ovum is produced, and females are born with all their ova present in their ovaries.
Each chromosome pair segregates independently of all other pairs according to the principle of
independent assortment. Many different combinations of chromosomes could result from the meiosis of a
single germ cell. Each parent can produce far more than 223 -more than 8 million- different genetic
combinations.
Occasionally, a zygote will split into separate but identical cells, which then become two individuals:
monozygotic (identical) twins. They have identical genes. This is very rare.
Dizygotic (fraternal) twins: pairs that result when a mother releases 2 ova at the same time and each is
fertilized by a different sperm.
Autosomes: 22 pairs of human chromosomes that are identical in males and females. Sex is determined by
the 23rd pair called the sex chromosome. Fathers determine the sex of the children by whether an x-
bearing or y-bearing sperm fertilizes the ovum.
What do genes do?
1. call for the production of amino acids that are necessary for the formation and functioning of new
cells. (e.g. people with brown eyes have pigment melanin and blue/green eyes have genes that call
for less pigmentation.)
2. Genes guide cell differentiation. Making some cell parts for the CNS and other for circulatory etc.
Genes influence and are influenced by the biochemical environment surrounding them during
development.
3. Some genes are responsible for regulating the pace and timing of development. Specific genes
turned on and off by other regulatory genes.
, 4. Environmental factors clearly influence how genes function. Environmental influences combine
with genetic influences to determine how a genotype is translated into a particular phenotype. The
environment within a nucleus, internal environment around the cell and external environment.
5. Genes simply don’t code for human characteristics but they interact with the environment at many
levels.
How are genes expressed?
1) simple dominant-recessive inheritance
Genotype: the genes that one inherits
Phenotype: one’s observable or measurable characteristics
Alleles: alternative forms of a gene that can appear at a particular site on a chromosome. Mendel
contributed to single pair inheritance by crossbreeding different strains of peas. His major discovery was a
predictable pattern to the way which two alternative characteristics appeared in offspring of cross
breeding.
dominant: a relatively powerful gene is expressed phenotypically and masks the effects of a less powerful
gene, capital letter
recessive: a less powerful gene that’s not expressed phenotypically when paired with a dominant allele,
lowercase letter
If we know the genetic makeup of parent, we can predict the percentage of children in a family who are
likely to display or carry a trait.
3 possible genotypes: AA, aa, Aa
AA and aa are homozygous for that attribute.
Aa is heterozygous, this person has been inherited alternative forms of the allele.
If each parent is heterozygous for an attribute, then they are carriers of the recessive allele and have 1/4
possibility of having a child with the recessive homozygous of that attribute.
Carrier: a heterozygous individual who displays no sign of a recessive allele in his phenotype but can pass
this gene to offspring.
Modifier Gene: genes that enhance or dilute the effect of other genes. E.g. children with PKU have light
hair and blue eyes.
Punnett Square: graphic representation of parents’ alleles and their possible combinations to from unique
inheritable traits.
2) Codominance (Incomplete Dominance): Condition in which two heterozygous but equally powerful
alleles produces a phenotype in which both genes are fully and equally expressed. E.g. blood types
A and B are codominant, that’s why we have AB blood type.
Another type of codominance occurs when one of two heterozygous alleles is stronger than the other but
fails to mask all of the other’s effects. The sickle cell trait is an example. 8% of African Americans carry the
recessive ‘sickle cell’ allele yet symptoms are rarely experienced. People who inherit two recessive sickle
cell genes develop the sickle cell anemia that causes insufficient oxygen distribution at all times. Many
who suffer from this die during childhood.
3) Sex-Linked Inheritance: some traits are called sex-linked characteristics because they are
determined by genes located on the sex chromosomes. Majority of these are produced by recessive
genes found only on X chromosomes. Males are more likely to inherit these recessive x-linked
traits. Females can be carriers but males don’t have that option, if they have the recessive allele
they’ll have the disease e.g. color blindness.
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