Introduction to Genetics
• Genetics- scientific study of heredity
• Gregor Mendel- father of genetics, laid the
foundation of the science of genetics
– Used garden peas for his experiments
• He crossed purebred plants with different
characteristics and study the results
Genes and Dominance
• Mendel studied seven different pea plant traits
• Trait- distinguishing characteristics that are
inherited, such as seed color or plant height
• P (parental) generation-original pair of plants
• F₁ (first filial) generation- offspring of original pair
of plants
• What did Mendel notice with the offspring?
– All F₁ offspring had the characteristic of only one of
the parents
• What did Mendel conclude about inheritance?
– Traits are inherited through the passing of factors
from parents to offspring
– Genes- piece of DNA that directs a cell to make a
certain protein
• Each gene has a specific position on a pair of
homologous chromosomes
– The traits studied were controlled by one gene
that occurred in two contrasting forms
– Alleles- different forms of a gene occurring at
same location on the chromosome
Alleles
• Each parent donates
one allele for every
gene
– Homozygous- alleles are
identical to each otherTT, tt
– Heterozygous- alleles are
different from each
other- Tt
• Genes influence the development of traits
– Genome- all of an organism’s genetic material
– Genotype- makeup of a specific set of genes
– Phenotype- physical expression of a trait, physical
characteristics
• Some alleles are
dominant, some are
recessive
• Dominant alleleexpressed as phenotype
when at least one allele is
dominant
– Uppercase Letters
• Recessive alleleexpressed as phenotype
only when two copies are
present
– Lowercase letter
Segregation
• He allowed F₁ plants to self-pollinate – produced F₂
generation
– F₁ generation, all plants had purple flowers
– F₁ plants are all heterozygous
– Some of the F ₂ generation, some plants had purple flowers some
had white
• Results of F₂- traits controlled by recessive allele
represented one fourth of F₂ plants
– Segregation- separation of alleles
– Gametes- sex cells
• Law of Segregation
– Organism inherit two copies of each gene, one from each parent
– The two copies segregate during gamete formation
• Each F₁ plant produces two types of gametes, those with
allele for tallness and those with allele for shortness
– Capital T represents a dominant allele, lowercase t represents
recessive allele. Result= F₂ generation with new combination of
allele
Punnett Squares
• The inheritance of traits
follows the rules of probability
• Punnett squares illustrate
genetic crosses
– Punnett squares- grid system
for predicting all possible
genotypes resulting from a
cross
• Axes represent the possible
gametes of each parent
• The boxes show possible
genotypes of the offspring
• Punnett squares can be used
to predict and compare the
genetic variations that will
result from a cross
Exploring Mendelian Genetics
• Dihybrid cross involves two traits
• Does the gene that determines whether a seed is
round or wrinkled in shaped have anything to do with
the gene for seed color?
• Mendel’s crossed plants that produced only round
yellow peas (genotype YYRR) with plants that produced
wrinkled green peas (genotype yyrr)
– All offspring produced round yellow peas
– Yellow and round are dominant over green and wrinkled
• Mendel allowed the F₁ generation to self-pollinate to
produce an F₂ generation
• What did he find out?
– Mendel’s dihybrid
crosses with
heterozygous plants
yielded a 9:3:3:1
phenotypic ratio
• What does this mean?
– Law of independent assortment
• Allele pairs separate independently of each other
during meiosis
• The alleles for seed shape segregated independently of
those for seed color
• Genes for seed shape and seed color in pea plants do
not influence each other’s inheritance
Traits and Probability
• Heredity patterns can be calculated with
probability
– Probability- likelihood that something will happen
– Probability predicts an average number of
occurrences, not an exact number of occurrences
– Probability applies to random events such as
meiosis and fertilization
Process of Meiosis
• How are new cells made?
– Mitosis
– Meiosis
• Meiosis- produces sex
cells (gametes)
– Reduces chromosome
number and creates
genetic diversity
– Diploid cells undergo two
cell divisions that result in
4 haploid cells
Meiosis
• You have two types of cells
– Body cells- somatic cells
– Germ cells- develop into gametes
• Located in ovaries and testes
• Gametes- sex cells: egg and sperm
• Gametes have DNA that can be passed to offspring
• Gametes- have half the number of
chromosomes that body cells have
Chromosomes and Meiosis
• How many chromosomes
does a normal human have?
– Body cells- 23 pairs; 46 total
• Homologous pairs of
chromosomes have same
structure
• 2 chromosomes- one inherited
from mother and one
inherited from father
• Autosomes- chromosome
pairs 1-22
• Sex chromosomes- pair 23,
X and Y
Chromosomes and Meiosis
• Body cells are diploid;
gametes are haploid
• Diploid (2n) cells have
two copies of every
chromosome
– Half the chromosomes
come from each parent
Chromosomes and Meiosis
• Haploid (n) cells have
one copy of every
chromosome
– Gametes are haploid
– Gametes have 22
autosomes and 1 sex
chromosome
Chromosomes and Meiosis
• Meiosis makes haploid cells from diploid cells
– Meiosis occurs in sex cells
– Meiosis produces gametes
Process of Meiosis
• 2 rounds of division in
meiosis
– Meiosis I and meiosis II each
have four phases, similar to
those in mitosis
– Pairs of homologous
chromosomes separate in
meiosis I
– Homologous chromosomes
are similar but not identical
– Sister chromatids divide in
meiosis II
– Sister chromatids are copies
of the same chromosome
Meiosis I
Meiosis II
Meiosis and Genetic Variation
• Crossing over during meiosis increases genetic diversity
– Crossing over is the exchange of chromosome segments
between homologous chromosomes
– Occurs during prophase I of meiosis I
– Results in new combination of genes
Meiosis and Genetic Variation
• Chromosomes contain
many genes
– Farther apart two genes
are located on a
chromosome, the more
likely they are to be
separated by crossing over
– Genes located close
together on a chromosome
tend to be inherited
together- Gene linkage
Meiosis
• Where are genes located?
– On chromosomes in cell nucleus
• Fruit fly, Drosophila, has 8 chromosomes
– 4 from male parent, 4 from female parent. These sets of chromosomes
are homologous- each of the 4 chromosomes that came from the male
parent has a corresponding chromosome from the female parents
• Diploid- cell that contains both sets of homologous chromosomes
– Represented by 2N
– For Drosophila, the diploid number is 8, 2N=8
– Diploid cells contain 2 complete sets of chromosomes and 2 complete
sets of genes
• Haploid- gametes of sexually reproducing organisms, contain only
single set of chromosomes, only a single set of genes
– For Drosophila, haploid number is 4, N=4
Phases of Meiosis
• Meiosis- process of reduction division in which
the number of chromosomes per cell is cut in
half through the separation of homologous
chromosomes in a diploid cell
– 4 haploid cells are genetically different from one
another and from the original cell
– Involves two distinct divisions- meiosis I and
meiosis II
– Produces gametes
Meiosis I
• Interphase, prophase, metaphase, anaphase, telophase
• Interphase I- each chromosome is replicated
• Prophase I- each chromosome pairs with its
corresponding homologous chromosome to form a
tetrad
– 4 chromatids in tetrad
– Crossing over- chromosomes exchange portions of
chromatids
• Produces new combinations of alleles
• Telophase I and Cytokinesis- Nuclear membranes form,
cells separate into two cells
Meiosis II
• After meiosis I, the two cells have sets of chromosomes and
alleles that are different from each other and from diploid
cells
• Prophase II- meiosis I results in two haploid daughter cells,
each with half the number of chromosomes as original cell
• Metaphase II- chromosomes line up in a similar way to the
metaphase stage of mitosis
• Anaphase II- sister chromatids separate and move toward
opposite ends of the cell
• Telophase II and Cytokinesis- meiosis II results in four
haploid daughter cells
• Four daughter cells contain just 2 chromosomes each
• Meiosis
Gamete Formation
• In males, meiosis results in four equal sized
gametes called sperm
• In females, only one large egg cell results from
meiosis
– Other three cells called polar bodies are not involved
in reproduction
• How is mitosis different than meiosis?
– Mitosis results in the production of two genetically
identical diploid cells, whereas meiosis produces four
genetically different haploid cells
• Comparison of Meiosis and Mitosis
Dominant and Recessive Alleles
• Some alleles are neither dominant nor recessive, and many traits are
controlled by multiple alleles or multiple genes
• Incomplete dominance- one allele is not completely dominant over
another
– Red plants (RR) crossed with white plants (WW)- pink (RW)
• Codominance- both alleles contribute to the phenotype
– Black feathered chicken (BB) crossed with white (WW)- all speckled offspring
(BBWW)
• Multiple alleles- many genes have more than two alleles
– Human genes for blood type
• Polygenic traits- traits controlled by two or more genes
– Variation in skin color
• Applying Mendel’s Principles- Thomas Hunt Morgan uses fruit flies to test
Mendel’s principles
– Produce large # of offspring
Sec 5- Linkage and Gene Maps
• Thomas Hunt Morgan- researched fruit flies in
1910 and discovered that chromosomes assort
independently, not individual genes
– Each chromosome is actually a group of linked genes
• How did Mendel manage to miss gene linkage?
– Six of the seven genes he studied are on different
chromosomes, the two genes found on same
chromosome are so far apart they assort
independently
Gene Maps
• If two genes are found on the same chromosome, does
this mean that they are linked together forever?
– Crossing over during meiosis separates genes on same
chromosomes
– What does this lead to?
• Genetic diversity
• Alfred Sturtevant- created gene map showing the
relative locations of each known gene on one of the
Drosophila chromosomes
– He showed genes close to each other on chromosome are
usually inherited together
– His method has been used to construct genetic mapsincluding maps of human genome