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Genetics
Mendelian Genetics
Genetic Engineering
Gregor Mendel
• Used pea plants to experiment on genetic
traits
• Pea plants can self-pollinate, producing
purebred offspring. The offspring had the
same characteristics as the parent
• Mendel cross-pollinated pea plants,
fertilizing the egg and sperm from different
plants, to produce hybrids.
Dominance
• Trait – a specific characteristic, such as
seed color or plant height, that varies from
one individual to another.
• Gene – the chemical factors that
determine traits.
• Alleles – the different forms of a gene.
– Example: Gene = hair color, Allele = brown
hair
Dominance
• Mendel discovered the principle of
dominance – some alleles are dominant
and others are recessive.
• Dominant – only one allele of the trait is
required for the trait to appear.
• Recessive – both alleles of the trait are
required for the trait to appear.
Dominance
Segregation
• The separation of alleles during meiosis
during the formation of the gametes.
• When the parent plant produces gametes,
the two alleles segregate from each other
so that each gamete carries only a single
copy of each gene. Therefore, each plant
produces two types of gametes.
Segregation
• P = parental
generation
• F1 = 1st filial
generation
• F2 = 2nd filial
generation
Segregation
Probability
• The likelihood that a particular event will
occur.
• Used to predict the outcomes of genetic
crosses.
• Punnett Squares – used to determine the
probability that a trait will occur.
Punnett Square
Punnett Square
• Homozygous – organisms that have two
identical alleles for the same trait
– Ex. BB or bb
• Heterozygous – organisms that have two
different alleles for the same trait
– Ex. Bb
• Phenotype – the physical characteristics
represented by the alleles
– Purple coloring
• Genotype – the genetic makeup
– BB
Punnett Squares
• Which alleles are homozygous?
– XX
• Which alleles are heterozygous?
– XY
• What is the genotype?
– XX and XY
• What is the phenotype?
– Male and female
Punnett Squares
• What is the percentage of the genotype?
– XX = 2/4 = 50%
– XY = 2/4 = 50%
• What is the percentage of the phenotype?
– Female = 2/4 = 50%
– Male = 2/4 = 50%
Punnett Square
• What is the percentage of genotype and
phenotype for a heterozygous brown-eyed
male and a heterozygous brown-eyed
female? B = brown, b = blue
• Genotype = 25% BB, 50% Bb, 25% bb
• Phenotype = 75% brown, 25% blue
Law of Independent Assortment
• Genes from different traits can
segregate independently during
the formation of gametes.
Independent assortment helps
account for the many genetic
variations observed in plants,
animals, and other organisms.
Law of Independent Assortment
Summary of Mendel’s Principles
• The inheritance of biological
characteristics is determined by individual
units called genes. Genes are passed
from parents to offspring.
• In cases in which two or more forms of the
genes for a single trait exist, some forms
of the gene may be dominant and others
many be recessive.
Summary of Mendel’s principles
• In most sexually reproducing organisms,
each adult has two copies of each gene,
one from each parent. These genes are
segregated from each other when
gametes are formed.
• The alleles fro different genes usually
segregated independently of one another.
Exceptions
• Incomplete dominance – the heterozygous
phenotype is expressed somewhere in
between the two homozygous phenotypes.
Exceptions
• Codominance – both alleles contribute to
the phenotype
Exceptions
• Multiple Alleles – Genes having more than
two alleles
Exceptions
• Polygenic traits – traits controlled by two
or more genes
Exceptions
• Pleiotropy – one gene that affects many
traits
Sex – linked inheritance
• Traits that are located on the sex
chromosomes are sex – linked.
• Most traits are found on the X
chromosome because it is much larger.
• Muscular dystrophy, hemophilia,
colorblindness, and melanoma are
diseases linked to the sex chromosomes
Sex-linked characteristics
• What are the genotypic and phenotypic
ratios for a normal male and a carrier
female of hemophilia?
• What are the genotypic and phenotypic
ratios for a colorblind male and a carrier
female of colorblindness?
• What are the genotypic and phenotypic
ratios for a normal male and a bald
female?
Genetic Engineering
• Humans have always genetically altered
the offspring of plants and animals.
• Selective breeding: breeding only
organisms with desired characteristics.
– Hybridization: crossing dissimilar organisms
to gain the best characteristics
– Inbreeding: breeding to similar organisms to
keep particular characteristics
• Breeding can increase genetic variation by
introducing mutations.
Genetic Engineering
• Cell transformation: a cell takes in DNA
from outside the cell, which becomes a
component of the cell’s DNA
• Transgenic organisms: cells of an
organism contain the genetic components
of a different organism
• Cloning: a member of a population of
genetically identical cells produced from a
single cell