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Genetics
“The book of the genealogy of Jesus Christ, the Son of David, the Son of Abraham”
Matt 1:1
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genetics: study of the inheritance of traits, passed from parents to offspring
genes: units of genetic information in the cell chromosomes
mitosis: process by which cell divides to form two new daughter cells; asexual
reproduction requires only mitotic cell divisions
differentiation: certain cells called out to become specialized components of tissues
and organs
Sexual Reproduction
• mating must occur between male and female of the same kind; heredity of both
parents combined to provide heredity of offspring
• male and female contribute equal amounts of genetic material in the form of DNA
• in humans, each somatic cell (body cell) has 46 chromosomes
• gametes (sex cells) contain 23 chromosomes (haploid set)
• fertilization: egg and sperm unite and produce zygote (diploid set)
• [Fig. 23.3 p. 493]
Meiosis
Mitosis
Description
Mitosis is a process of asexual
reproduction in which the cell
divides in two producing a replica,
with an equal number of
chromosomes in haploid cell
Type of
Asexual
reproduction
Function
Meiosis is a type of cellular
reproduction in which the number of
chromosomes are reduced by half
through the separation of
homologous chromosomes in a
diploid cell.
Sexual
Cellular reproduction and general
Sexual reproduction
growth and repair of the body
Takes place in All organisms
Humans, animals, plants, fungi
Types of cells
Within somatic cells (cells that
make up the body)
Takes place within gamete cells (sex
cells).
Number of
divisions
Mitosis undergoes only one
division.
Meiosis undergoes two divisions.
Produces
Two identical daughter cells
Four meiotic products or haploid
gametes
Mixing of
Mixing of chromosomes cannot
Mixing of chromosomes can occur.
chromosomes occur.
Number of
Same as parent cell
chromosomes
Half of the original gamete cell before
meiosis.
Origin of Modern Genetics
• Gregor Mendel (mid-1800s): experimented with pea plants to discover how organisms
inherit their traits
• purebred: traits have remained the same for many generations
• cross: mating purebred organisms with different traits to test how traits are inherited;
placed pollen of tall pea plant on stigma of short pea plant; pollen and egg unite to
produce seed with factor from tall and short plant
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Mendel crossed purebred tall plants with purebred short plants (parent generation)
F₁ - first offspring generation; hybrid: ancestors not alike; all tall
F₂ - hybrid tall pea plants pollinated themselves; 3/4 tall + 1/4 short
parent generation
F₁ - first offspring generation
F₂ - second offspring
generation
purebred + purebred
hybrid + hybrid
purebred hybrid hybrid
purebred
TT (tall) x tt (short)
Tt (tall) x Tt (tall)
TT x Tt x Tt x tt
each offspring had two factors (alleles) for each trait, one from each parent; if factors
were the same, plant was purebred for that trait; if they were different, plant was
hybrid
each factor is dominant (more influence) or recessive (less influence)
when both factors dominant = dominant trait shown (homozygous)
when both factors recessive = recessive trait shown (homozygous)
one dominant + one recessive = dominant shown, recessive masked (heterozygous);
law of dominance
phenotype: the way a plant or organism looks; physical characteristics (Tt and tt have
same phenotype)
genotype: specific factors an organism possesses (Tt and tt have different genotype)
Punnett Squares
• symbols used to represent genes; boxes represent genetic possibilities
• [Fig. 23.5 p. 497]
• top: all possible gametes of one parent/side: all possible gametes of other parent;
gametes can contain only one allele for each trait
• form crosses by filling in each box with one allele from one parent on top and one
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parent on side
law of segregation: alleles for each trait segregate or separate randomly during the
formation of gametes
codominance: mixing or blending of traits [Fig. 23.9 p. 498]
law of independent assortment: many traits inherited independently of one another;
segregation of one gene does not influence segregation of another
linkage: two genes located on same chromosome which may be inherited together;
do not follow law of independent assortment (Hemophilia A + colorblindness)
dihybrid cross [Fig. 23.11 p. 499]
Sex Chromosomes
• all normal humans have 23 pairs of chromosomes; each pair responsible for certain
functions, but pair #23 determines gender and has distinct appearance in each sex
• females: XX
• males: XY
• --> male contributes one of two chromosomes which determines gender of offspring
depending on whether that offspring receives an X (female) or Y (male) chromosome;
mother always contributes X
• sex-linked traits: genes located on the sex chromosome; inherited trait that has the
gene on the X chromosome
• [Fig 23.14 p. 501]
• also colorblind example:
• parent generation: XgY + XGX and XY + XX (where g is recessive colorblindness)
• F₁: XGXg (carrier) + XGY
• F₂: XgY --> colorblind
• female will be colorblind only if she inherits two recessive alleles (XgXg) so less likely
than males (XgY); females 1/250, males 1/12
Human Genetics
• gene frequency: extent to which a certain gene exists in a population
• polydactyly is a dominant allele but not many people have 6 fingers so gene
frequency low
• multiple allele inheritance: >2 different alleles of a gene for a particular trait
• ex: blood type [Table 23.4 p. 504] - type O or AB has only one genotype; types A or B
can have different genotypes
• pleiotropy: most genes influence several unrelated traits (ex: sickle cell anemia, where
homozygous individuals have anemia and heterozygous have protection agains
malaria)
• polygenic inheritance: traits controlled by many different pairs of genes (skin color,
height
• sex-linked disorders: hemophilia [Fig 23.17 p. 504]
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eugenics: attempt to improve human race through control of hereditary factors
Homework
Section 23.1 #1-3
Section 23.2 #1-7
Section 23.3 #1-4
Finish Genetics lab (p. 159-165)