Transcript Intor to Genetics n Meiosis

Introduction to Genetics
&
Meiosis
HONORS BIOLOGY
Genetics – study of heredity
• Heredity – passing on characteristics from
parents to offspring
• Traits – inherited characteristics
Gregor Mendel
• 19th Century Austrian Monk
• First important studies of heredity
Monohybrid Crosses
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Two parents, differing by a single trait
Parents = P1
First Generation is called F1 – Filial 1
Second Generation is F2
How information is transferred
• Gametes – sex cells
• Fertilization – uniting male and female sex
cells
• Genetic information is located in genes,
which are located on chromosomes.
• Each trait is represented by two alleles.
• Rule of Unit Factors-one factor(allele)
from each parent
Rule of Dominance
• An allele can carry either a Dominant or a
Recessive trait.
• For a recessive trait to be “expressed” ,
both alleles must carry it.
• A dominant trait is represented by a capital
letter, recessive by a lower case.
Law of Segregation
• The two alleles for a trait separate when a
gamete is formed.
• The parent passes on, at random, one allele
to each off-spring.
Gene expression
• Phenotype – the way an organism “looks”.
• Genotype – the actual gene combination an
organism carries.
• Homozygous – two alleles for a trait are
the same.
• Heterozygous – two alleles for a trait are
different.
B
b
B
BB
bB
b
Bb
bb
Punnett Squares of all Possible crosses:
BB x BB
B B
BB x Bb
B B
Bb x Bb
B b
Bb x bb
B b
bb x bb
b b
B BB BB B BB BB B BB bB b Bb bb b bb bb
B BB BB b Bb Bb b Bb bb b Bb bb b bb bb
PART II
MEIOSIS
Diploid Cell
• Cell that contains two of each chromosome
• Supports Mendel’s conclusion that
organisms have two factors (alleles) for
each trait
• One allele is on each of the paired
chromosomes
Homologous Pairs
• Together the two chromosomes of each
pair in a diploid cell help determine what
the individual organism looks like.
• These paired chromosomes are called
homologous pairs or homologous
chromosomes.
• Have genes for the same traits arranged in
the same order, but there are different
possible alleles for the same gene.
• Two chromosomes in a homologous pair
are not identical
Haploid cells
• A cell with one of each kind of
chromosome - gamete
• Male gametes are called sperm
• Female gametes are called eggs
• When a sperm fertilizes an egg the
resulting off-spring is called a zygote
Meiosis
• Cell division which produces gametes
containing half the number of
chromosomes of the parent
• Two separate divisions - Meiosis I &
Meiosis II
• Meiosis I begins with 1 diploid cell and
Meiosis II ends with four haploid cells
Interphase
• Cell carries out its normal functions
• Sex cells replicate their chromosomes
• Each chromosome then consists of two
identical sister chromatids held together by
a centromere
Prophase I
• Chromosomes coil up and spindle forms
• Each pair of homologous chromosomes
comes together top form a four-part
structure called a tetrad
• Pair so tightly that crossing-over can occur
• resulting in new allele combinations
Metaphase I
• Tetrads line up on mid-line of spindle
Anaphase I
• Homologous chromosomes separate and
move to opposite ends of the cell
Telophase I
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Spindle is broken down
Chromosomes uncoil
Cytoplasm divides to form two new cells
each cell has only half the genetic
information of the original cell because it
has only one chromosome from each
homologous pair
Second Phase of Meiosis
• The newly formed cells must divide again
to create the haploid cells
Comparing Meiosis and Mitosis
Chromosome behavior
Mitosis: Homologous chromosomes independent
Meiosis: Homologous chromosomes pair forming bivalents
until anaphase I
Chromosome number- reduction in meiosis
mitosis- identical daughter cells
meiosis- daughter cells haploid
Genetic identity of progeny:
Mitosis: identical daughter cells
Meiosis: daughter cells have new assortment of parental
chromosomes
Meiosis: chromatids not identical, crossing over