Transcript Meiosis

Chapter 11
Intro. to Genetics
Chap. 11-4
Meiosis
•
Mitosis – division of body cells
(somatic cells)
• End result - 2 cells identical to starting
cell w/same number of chromosomes
• Meiosis – production of sex cells (sperm
& egg) (gametes)
• End result – 4 cells with ½ the
chromosome number as starting cell
Karyotype
• A picture of one’s
chromosomes
arranged in pairs
• Each pair are alike
in that they carry
genes for the same
traits
• Are called homologous chromosomes
Homologous Pair
Chromosome Number
• Gametes – called haploid b/c they
contain only half the number of
chromosomes
• Called the n number (n refers to number of
chromosomes in gametes)
• Gametes have only one member of each
homologous pair
Chromosome Number
• Somatic cells – called diploid b/c they contain
two of each chromosome or 23 pairs (in
humans)
• Each pair – called a
homologous pair
• Because each pair forms a
4-stranded group, they are
also called a tetrad
• Come together in Prophase I in
a process called synapsis
Divisions of Meiosis
• Meiosis occurs in 2 major divisions:
• 1st division – Meiosis I - separates
homologous pairs of chromosomes
(reduction phase)
• 2nd division – Meiosis II - separates
chromatids of each chromosome (division
phase) & is similar to mitosis
Meiosis
Meiosis
• Important events to remember:
• Synapsis & crossing over – Prophase I
• Separation of homologous
chromosomes – Anaphase I
• Division of chromatids – Anaphase II
Importance of Meiosis
• Causes variation in chromosomes by:
• 1. Crossing over - when homologous
chromosomes
exchange genes
Importance of Meiosis
• 2. Usually results in random segregation
(separation)
of chromosomes
Both result in new
genetic combinations in the offspring
Mitosis vs. Meiosis
Mitosis
Meiosis
1 division
2 divisions
Produces 2 genetically
identical cells
Cells are diploid
Produces 4 genetically
different cells
Cells are haploid
Produces body
(somatic) cells
Produces sex cells
(gametes)
Meiosis in Males
Meiosis in Females
CHAP. 11-1 & 11-2
Mendel’s Laws of Heredity
• Genetics – study of heredity
• Heredity – passing of traits from parent
to offspring
• Traits transmitted by genes located on
chromosomes
• Most traits determined by a pair of
genes located on paired chromosomes
(homologous)
Mendel’s Laws of Heredity
• Gregor Mendel – “Father of
Genetics”
• Studied garden pea plants;
2 reasons:
1.Have 7 contrasting traits
• ex. – tall vs.
short, green pod
vs. yellow, etc.
Mendel
• 2. Easy to cross-pollinate (the transfer of male pollen
grains from one plant to the
female organ of another plant)
(usually pea plants selfpollinate
Genetics Vocab.
• Genotype – genetic makeup of an
organism (Tt)
• Phenotype – physical makeup of an organism
(tall)
• Homozygous (pure) – when 2 genes for a trait
are the same (TT or tt)
• Heterozygous (hybrid) – when 2 genes for a
trait are different (Tt)
• Monohybrid cross – one with only 1 trait
crossed
• Alleles – different forms of a gene (T or t)
Mendel’s Experiments
• Mendel first crossed pure tall plant with
pure short plant
• To work a genetics problem:
• Step 1- Pick symbols:
• T - tall, t - short
• Step 2 - Diagram the cross:
• TT x tt (Parents or P generation)
Mendel’s Experiments
• Step 3 – Do the Punnett square:
• TT x tt
• offspring are called 1st filial or F1
generation)
• one trait shows up (tallness) & one
disappears (shortness)
Mendel’s Experiments
• Step 4 – List ratios:
• Genotypic ratio –
• Phenotypic ratio -
Mendel’s Experiments
• Next Mendel did a second generation cross
(cross 2 plants from the 1st generation)
• F2 cross = Tt x Tt
• Now the tall trait appears
• Offspring are called F2 generation
Monohybrid Crosses
Mendel’s Laws
• Law of Dominance – in an
organism w/contrasting alleles
(different forms of a gene), one
gene shows up & the other disappears
• Gene that shows up – dominant
• Gene that disappears – recessive
• Law of Segregation – pairs of genes separate
when gametes are formed in meiosis (shown in
the Punnett square
Ch. 11-3 Mendel’s Laws
• Law of Independent Assortment – pairs
of genes separate independently of each
other
• Dihybrid cross – one with 2 different
traits
Dihybrid Cross – used to show Mendel’s Law
of Independent Assortment
In guinea pigs, black fur is dominant over white. Rough coat is
also dominant over smooth. Cross a homozygous black, rough
guinea pig with a heterozygous black, rough guinea pig.
•F-o-i-l (first – outside – inside - last)
Incomplete Dominance
• Incomplete dominance – when 1 gene is not
completely dominant over the other; a blending of traits
occurs
• Problem - In some flowers, red is incompletely
dominant over white. The heterozygous condition
produces pink flowers.
• Pick symbols: R – red, R′ - white
• Possible genotypes:
• RR- red
• R′R′ - white
• RR′ - pink
Codominance
• Codominance – when neither gene is dominant;
both genes in a pair are expressed
• In cattle, red hair is codominant with white. The
heterozygous condition produces a roan (mix of
red & white) offspring.
• Pick symbols – R-red; W-white R
• Possible genotypes:
• RR – red
• WW – white
• RW - roan
Bio. Bingo
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Mendel
Independent assortment
Heterozygous
Genotype
Phenotype
Genetics
Heredity
Recessive
1st filial
Homozygous
Peas
Traits
Genes
Punnett square
alleles
segregation
F2
self-pollination
dominant
monohybrid
incomplete dominance
codominance
1:2:1
cross pollination
dominance
dihybrid
dominant
3:1
gametes
hyrbid
fertilization
probabilityb