Transcript Ch.11 - Jamestown Public Schools

Unit 4 Genetics
Ch. 11 Introduction to Genetics
The Work of Gregor Mendel
Genetics - the scientific study of
heredity
Mendel is considered the “Father” of
genetics
Gregor Mendel’s Peas
Mendel was an Austrian monk, that
worked on pea plants
His experiments with pea plants laid the
foundation of the science of genetics
Gregor Mendel’s Peas
Fertilization - when male & female sex
cells join together
Pea flowers are normally selfpollinating, meaning the sperm cells in
the pollen fertilize the egg cells in the
same flower
Gregor Mendel’s Peas
Seeds produced by self-pollination
inherit all of their characteristics from
the single plant that bore them
True-breeding - if plants were allowed
to self-pollinate, they would produce
offspring identical to themselves
Gregor Mendel’s Peas
Mendel wanted to produce seeds by
joining male & female sex cells from 2
different plants
He cut off the pollen (male) parts of a
plant, & dusted pollen from another
plant onto the flower (female)
Gregor Mendel’s Peas
Cross-pollination - produces seeds that
had 2 different parent plants
Genes & Dominance
P - parent generation
F1 - First generation (first generation of
offspring)
F2 - Second generation (offspring from
the F1 generation)
Genes & Dominance
Trait - a specific characteristic
Ex.) seed color, plant height
Hybrids - offspring of crosses between
parents with different traits
Ex.) cross between plant with yellow seed
color & plant with green seed color
Genes & Dominance
Mendel concluded that biological
inheritance is determined by factors
that are passed from 1 generation to
the next
Genes - chemical factors that determine
traits
Genes & Dominance
Alleles - different forms of a gene
Ex.) gene for plant height occurs in 1 form
that produces tall plants & in another form
that produces short plants
Mendel’s 2nd conclusion is the principle
of dominance
Genes & Dominance
The principle of dominance states that
some alleles are dominant & others are
recessive
Dominant allele for a trait will always be
exhibited (expressed or shown)
Recessive allele will only be expressed
when a dominant allele is not present
Mendel’s Seven F1 Crosses on
Pea Plants
Segregation
Gametes - sex cells (sperm or egg)
Segregation - during gamete formation,
alleles segregate (separate) from each
other so each gamete only carries a
single copy of each gene
Segregation
Therefore, each F1 plant produces 2
types of gametes, those with the allele
for tallness & those with the allele for
shortness
Punnett Squares
Punnett square - a diagram that might
result from a genetic cross
Punnett squares can be used to predict
& compare the genetic
variations that will result
from a cross
Punnett Squares
Homozygous - organisms that have 2
identical alleles for a particular trait
Ex.) TT or tt
Heterozygous - organism that
has 2 different alleles for the
same trait
Ex.) Tt
Punnett Squares
Phenotype - physical characteristics
Ex.) Tall plants
Genotype - genetic makeup
Ex.) TT
Independent Assortment
Independent assortment - genes for
different traits can segregate (separate)
independently during gamete formation
Independent assortment increases
genetic variation (genetic
diversity, helps create
genetically different organisms)
A Summary of Mendel’s
Principles
1. The inheritance of biological
characteristics is determined by
individual units - genes
Genes are passed from parents to their
offspring
A Summary of Mendel’s
Principles
2. In cases where 2 or more forms
(alleles) of the gene for a single trait
exist, some forms of the gene may be
dominant & others may be recessive
A Summary of Mendel’s
Principles
3. In most sexually producing
organisms, each adult has 2 copies of
each gene (1 from each parent)
These genes are segregated (separated)
from each other when gametes are formed
A Summary of Mendel’s
Principles
4. The alleles for different genes usually
segregate (separate) independently of 1
another
Beyond Dominant & Recessive
Alleles
Some alleles are neither dominant nor
recessive, & many traits are controlled
by multiple alleles or multiple genes
Beyond Dominant & Recessive
Alleles
Incomplete dominance - when 1 allele
is not completely dominant over
another
The heterozygous phenotype
is somewhere in between the
2 homozygous phenotypes
Beyond Dominant & Recessive
Alleles
Codominance - where both alleles
contribute to the phenotype
Flowers would not be pink, (a blend of red
& white), but both red & white speckled
Beyond Dominant & Recessive
Alleles
Multiple alleles - when genes have more
than 2 alleles
It does not mean that an individual can
have more than 2 alleles
It only means that more than 2 possible
alleles exist in a population
Multiple Alleles
Beyond Dominant & Recessive
Alleles
Polygenic traits - traits controlled by 2
or more genes
Ex.) at least 3 genes are responsible for
making the reddish-brown pigment in the
eyes of fruit flies
Genetics & the Environment
The characteristics of any organism are
not determined solely by the genes it
inherits
Characteristics are determined by
interaction between genes & the
environment
Genetics & the Environment
Ex.) genes may affect a sunflower
plant’s height & the color of its flowers
However, these conditions are also
influenced by climate, soil conditions, &
the availability of water
Ex.) Rabbit fur color in winter &
summer
Chromosome Number
All cells of an organism (except for sex
cells, gametes) have the same # of
chromosomes
Each body cell has 2 sets of
chromosomes
Homologous chromosomes - the same
chromosomes, 1 set from each parent
Homologous Chromosomes
Chromosome Number
Diploid - (2n) - a cell that has both sets
of homologous chromosomes
Haploid - (n) - a cell that has half the
normal set of chromosomes, or 1 set
(only sex cells are haploid)
Phases of Meiosis
Meiosis - process of reduction division,
where the # of chromosomes per cell is
cut in 1/2, through the separation of
homologous chromosomes in a diploid
cell
Phases of Meiosis
During meiosis 1, crossing-over may
occur
Crossing-over - when
chromosomes exchange portions of
their chromatids
Phases of Meiosis
Crossing-over results in the exchange of
alleles between homologous
chromosomes & produces new
combinations of alleles
Crossing-over increases genetic
variation (genetic diversity, helps create
genetically different organisms)
Phases of Meiosis
Meiosis II, begins with 2 genetically
different haploid (n) cells, & results in 4
(n) genetically different haploid cells
Therefore, Meiosis II is a mitotic
division
Meiosis I
Meiosis II
Gamete Formation
In males, the haploid
gametes are sperm
In females, the haploid
gametes are eggs
Comparing Mitosis & Meiosis
Mitosis results in the production of 2
genetically identical diploid (2n) cells
Mitosis produces all cells of the body,
except sex cells
Meiosis produces 4 (n) genetically
different haploid cells
Meiosis produces ONLY sex cells (gametes)
Gene Linkage
Each chromosome is a group of linked
genes
It is the chromosomes, however, that
line up independently, not individual
genes (Principle of Independent
Assortment)