Transcript Chapter 11 – Introduction to Genetics

Chapter 11 – Introduction to
Genetics
Vocab: genetics, fertilization, true-breeding, trait,
hybrid, gene, allele, segregation, gamete, probability,
Punnett square, homozygous, heterozygous,
phenotype, genotype, independent assortment,
incomplete dominance, codominance, multiple alleles,
polygenic traits, homologous, diploid, haploid, meiosis,
tetrad, crossing-over, gene map
11-1 The Work of Gregor Mendel
Genetics is the scientific study of heredity.
Gregor Mendel
• Austrian monk in what is now the Czech
Republic in 1822
• Studied garden peas
Pea Plants
• Fertilization is when male and female
reproductive cells join.
• Male cell = pollen; Female cell = egg
• Pea plants are normally self-pollinating –
egg and pollen from same plant – one
parent
• True-breeding plants – after self-pollinating
they have identical offspring
• Mendel began manipulating the pollination
– cross-pollination
• Mendel studied seven different traits that
had two contrasting forms
• Trait = a specific characteristic that varies
from one individual to another
The offspring of crosses between parents
with different traits are called hybrids.
Mendel crossed plants with the two forms
• P: Parental Generation: Tall x Short
• F1: First filial Generation: all Tall
• F2: Second filial Generation:75% tall &
25% short
Made two conclusions:
• Inheritance is determined by factors
passed from one generation to the next
called genes. Genes have different forms
called alleles.
• The principle of dominance states that
some alleles are dominant and others are
recessive. Recessive alleles are masked
by dominant alleles.
• Segregation – When each F1 plant flowers
and produces gametes, the two alleles
segregate from each other so that each
gamete carries only a single copy of each
gene. Therefore, each F1 plant produces
two types of gametes – those with the
allele for tallness and those with the allele
for shortness
Segregation
11-2 Probability and Punnett
Squares
The likelihood that a particular event might
occur is called probability.
Coin landing on heads = 50%
Coin landing on heads three times in a row =
½ x ½ x ½ = 1/8
The principles of probability can be used to
predict the outcomes of genetic crosses.
Punnett squares can be used to predict and
compare the genetic variations that will
result from a cross.
• Homozygous = organism with two identical
traits; TT or tt
• Heterozygous = organisms with two
different traits; Tt
• Phenotype = physical characteristics; tall
or short
• Genotype = genetic makeup; TT, Tt, or tt
Punnett Square
Parent 1 genotype=TT
Parent 2 genotype=tt
Punnett Square
Parent 1 genotype=TT
Parent 2 genotype=tt
T
t
t
Tt
T
11-3 Exploring Mendelian Genetics
Independent Assortment = looks at the
inheritance of two traits
The principle of independent assortment
states that genes for different traits can
segregate independently during the
formation of gametes. Independent
assortment helps account for many
genetic variations observed in plants,
animals, and other organisms.
Beyond Dominant and Recessive
Alleles
• Some alleles are neither dominant nor
recessive, and many traits are controlled
by multiple alleles or multiple genes.
– Incomplete dominance
– Codominance
– Multiple alleles
– Polygenic traits
1. Incomplete dominance – flower color
(primrose, 4 o’clocks, snapdragon), horse
coat color, sickle-cell anemia
• 4 o’clocks:
– RR= red
– RW= pink
– WW= white
• Each dominant allele produces red
pigment: RW makes ½ as much and
appears pink
Incomplete Dominance
2. Codominance – both alleles contribute to
the phenotype
• Chickens – allele for black feathers is
codominant with white feathers…
heterozygote is an “erminette” speckled
chicken
• Human AB blood type
3. Multiple Alleles – more than two possible
alleles exist in a population
• Human ABO blood typing system
• Coat color in rabbits
4. Polygenic Traits – traits controlled by two
or more genes
• At least three genes control pigment in the
eyes of fruit flies
• The range of skin colors is from more than
four genes
Applying Mendel’s Principles
• 1900’s American geneticist Thomas Hunt
Morgan started studying fruit flies,
Drosophila melanogaster because it was
small, reproduce quickly, and have as
many as 100 offspring
Characteristics of any organism are
determined by interaction between genes
and the environment including climate,
soil, food availability, pollution.
11-4 Meiosis
• Chromosome number
• Genes are located on chromosomes in the
cell nucleus.
• Chromosomes are found in pairs called
homologous chromosomes.
– Fruit flies have 8 chromosomes and 4
homologous pairs; 4 from female and 4 from
male
• A cell with both sets of homologous
chromosomes is diploid
– Symbol = 2N; body cells are diploid
• A cell with one set of chromosomes is
haploid
– Symbol = N; gametes are haploid
Phases of Meiosis
• Meiosis is a process of reduction division
in which the number of chromosomes per
cell is cut in half through the separations of
homologous chromosomes in a diploid
cell.
Terms
• Tetrad = structure formed during meiosis in
which homologous chromosomes pair up.
• Crossing over = the 4 chromatids of a
tetrad may exchange portions resulting in
new combinations of alleles.
Mitosis results in the production of two
genetically identical diploid cells, whereas
meiosis produces four genetically different
haploid cells.