GENETICS
Genetics—the scientific study of heredity
Modern genetics is based on the knowledge that traits are controlled by genes located on
chromosomes.
CHARACTERISTICS OF MEIOSIS:
1.
Occurs only in gonads (ova and testes)
a. Males—testes
b. Females—ovaries
2.
Produces cells called gametes that have half the normal chromosome number for that species.
Haploid is the term used to describe the chromosome number in this type of cell.
3.
A two stage cell division.
a. Meiosis one—involves the formation of tetrads and is the reduction division, that is,
the division that makes the resulting daughter cells haploid. Chromosomes do not
break apart at the centromeres during the first meiotic division as they do in mitosis.
Crossing over may occur early in this stage of meiosis.
b. Meiosis two—involves the separation of the chromatids and formation of the actual
daughter-cells or gametes.
4.
Types of meiosis include:
a. Spermatogenesis—production of the male gametes in humans, called sperm. Four
full-sized daughter cells produced per mother cell (primary spermatocyte) in this type
of meiosis.
b. Oogenesis—production of the female gametes in humans, ova (eggs). Produces only
one full-sized daughter cell per mother cell (primary oocyte) in this type of meiosis.
5.
The union of a haploid sperm and a haploid ovum produces a diploid (containing the normal
number of chromosomes for a species) zygote which develops into a new individual of that
species.
MENDEL & MEIOSIS
Gregor Mendel—father of modern genetics. Austrian monk that discovered the basic principles of
heredity in his work with garden peas.
Why did Mendel choose peas?
1.
2.
3.
4.
5.
Readily available
Could be cultivated quickly
Produced large numbers of offspring
Easily cross/selectively bred
Displayed several traits in one of two contrasting forms
Mendel’s experiments:
1.
Cross-pollinated two plants with contrasting traits (P generation)
2.
One of the traits seemed to have disappeared in offspring (F1 generation)
3.
Cross pollinated members of the F1 generation
4.
Trait that had disappeared, presented itself once more in the offspring of the second
cross (F2 generation)
5.
Mendel reasoned that each trait was controlled by two “factors”, one being hidden in the
F1 generation. Today we call these factors genes.
Mendel’s three laws of inheritance:
Law of Dominance—one gene in a pair may prevent the other gene form being expressed.
This gene is said to be dominant. The gene that is hidden is said to be recessive. Terms
associated with this principle:
1.
Homozygous (purebred)—both genes in a pair identical.
2.
Homozygous dominant—two dominant genes in a pair
3.
Homozygous recessive—two recessive genes in a pair
4.
Heterozygous—one dominant and one recessive gene in a pair (hybrid)
Law of Segregation—two genes for the same trait segregate or separate during meiosis
(gamete formation).
Law of Independent Assortment—genes segregate independently of each other during meiosis
(gamete formation)
Terminology associated with Mendelian genetics:
1.
Allele—alternate gene form for each trait
2.
Genotype—combination of alleles present for a trait
3.
Phenotype—appearance of a trait as determined by a certain genotype
4.
Monohybrid cross—a cross involving the study of the inheritance of one trait
5.
Dihybrid cross—a cross involving the study of the inheritance of two traits
Incomplete dominance—occurs when neither of the alleles for a trait is dominant over the other.
In incomplete dominance, the traits seem to blend to form an intermediate form of the trait, such as
the pink flower of the four-o’clock plant.
Test cross—the cross of an organism with an unknown dominant genotype with an organism that is
homozygous recessive for that trait.
HUMAN GENETICS
How do scientists study human genetics?
1.
Population sampling—researchers use statistical rules to select a small group of
individuals that represent the whole population.
2.
Twin studies—identical twins used to distinguish between genetic and environmental
influences on specific traits.
3.
Pedigree studies—family record that shows how a trait is inherited over several
generations.
Human genetic traits:
1.
Single allele traits—traits controlled by a single allele of a gene.
Sickle-cell disease—allele A codes for normal hemoglobin; allele A’ codes for abnormal
hemoglobin. Alleles are codominant.
AA—normal hemoglobin and erythrocytes (RBC’s)
AA’—both normal and abnormal hemoglobin and
intermediate shaped cells
A’A’—have only sickle cells
Huntington’s disease—single allele trait caused by a dominant gene. Progressive
disease of the nervous system characterized by involuntary twitching movements of the
arms, legs, face, and body. Eventually may cause loss of muscle control, mental
illness, and death. Most carriers don’t know they have the disease until they pass it on
to their children. Recently geneticists have found a genetic marker for this disease.
2.
3.
Polygenic traits—trait controlled by two or more genes.
A.
Skin color—determined by the additive effect of four to
seven genes.
B.
Eye color—inherited in a manner similar to skin color.
Multiple allele traits—three or more alleles of the same gene
that code for a single trait.
Blood type is one example.
IA—codes for antigen A (codominant)
IB—codes for antigen B (codominant)
i—codes for no antigen (recessive)
4.
Sex determination—in some organisms, one pair of homologous chromosomes is
different. These determine the sex of an individual, and are called sex chromosomes
(X & Y).
Male—XY
Female—XX
All other chromosomes in an organism are called autosomes.
5.
Sex-linked traits—alleles for these appear as recessive genes found only on the X
chromosome. Males have only one X chromosome. Since no complementary portion of
the Y chromosome exists, a single recessive allele on the X chromosome will be
expressed. Sex linked traits are found predominately in males.
A. Colorblindness—inability to distinguish red from green most common.
B. Hemophilia—lack of a protein necessary for blood clotting.
6.
Sex-influenced traits—male pattern baldness. Gene for
baldness (B) is dominant in males but recessive in females.
Influenced by hormones.
7.
Non-disjunction—caused by failure of chromatids to separate during meiosis II.
Monosomy—45 chromosomes
Trisomy—47 chromosomes
A.
Down Syndrome—extra chromosome on pair 21(trisomy 21).
B.
Klinefelter syndrome—trisomic genotype XXY. Males with this syndrome are
typically tall, and they may have small testes and slight breast development.
They also may have minor problems with learning and are usually infertile.
C.
Turner syndrome—monosomic genotype XO. Females
with this condition are typically short, with a thick, webbed
neck. They may have mild problems with learning, and
they usually are infertile because they lack normal
ovaries.
How are genetic disorders detected:
1.
Genetic screening—may involve a karyotype.
2.
Amniocentesis—amniotic fluid removed from uterus.
3.
Fetoscopy—fetus observed by camera.
4.
Ultrasound—high frequency sound waves bounced off fetus to form a picture.
5.
Chorion villi sampling—tissue sample of chorion villi.
Three factors that contribute to genetic diversity in organsims:
1. Crossing over during synapsis
2. Independent orientation (Mendelian independent assortment)
3. Random fertilization