Chromosomes and
Meiosis
6.1, 6.2, 6.3
Essential Question: How does
reduction division produce haploid
gametes and spores.
Somatic cells are body cells that make up all of the
cells in your body that are not germ cells.
Soma is the Greek word for “body”.
Germ cells are cells in your reproductive organs –
ovaries and testes.
These cells develop into ova (eggs) and spermatozoa
(sperm).
Human cells have 23 PAIRS of Homologous
chromosomes (46 chromosomes in total).
Each homologous chromosome has a similar structure
and length with the other chromosome in the pairing.
The first 22 chromosome pairs (ordered from largest
to smallest) are called autosomes.
Autosomes are not related to sex determination or
sex-linked traits.
Chromosome pair 23 is a sex chromosome – directly
controls the development of sexual characteristics.
These sex chromosomes are either X chromosomes or
Y chromosomes. Yes…write that down.
An organism with two X chromosomes is female.
An organism with one X and one Y is male.
Sexual reproduction involves the fusion of gametes
resulting in offspring that are a genetic mixture of
both parents.
Fertilization is the actual fusion of an egg and a
sperm.
Your body cells (somatic cells) are diploid; they each
have two copies of each chromosome (one from mom
and one from dad).
Diploid cells can be represented by 2n. Diploid for
humans means having 46 chromosomes.
Gametes are haploid; they only have one copy of
each chromosome.
Haploid cells like egg and sperm can be represented
by n. They have 23 chromosomes in humans.
In the egg, the sex chromosome is always an X.
In the sperm, the sex chromosome can be either an X
or a Y.
Meiosis is a form of nuclear division that divides a
diploid cell into a haploid cell.
What’s the difference between mitosis and meiosis?
Mitosis produces most body cells (diploid cells).
Meiosis produces sex cells (haploid cells).
The Process of Meiosis
6.1, 6.2
Essential Question: What are the steps of
meiosis and what is the importance of crossing
over?
Meiosis is a form of nuclear division that creates 4
haploid cells from one diploid cell.
There are 2 rounds of cell division – meiosis 1 and
meiosis 2.
Meiosis I divides homologous chromosomes.
Meiosis I has 4 stages (PMAT) – prophase I,
metaphase I, anaphase I, and telophase I. Easy…right?
Meiosis II divides sister chromatids.
Meiosis II has 4 stages (PMAT)– prophase II,
metaphase II, anaphase II, and telophase II. Could it
be any easier?
REMEMBER…
Meiosis produces haploid cells.
MITOSIS produces DIPLOID cells.
The male gamete, sperm, is produced by meiosis.
The female gamete, egg, is produces by meiosis.
SOMATIC cells (body cells) are produced by MITOSIS.
Mendel and Heredity
6.3
SC.912.L.16.1
Essential Questions: What is the law of independent
assortment? What is the law of gene segregation.
What do you think of when you here the word trait?
Traits are distinguishing characteristics that are
inherited.
Genetics is the study of biological inheritance
patterns and gene variation within organisms.
What does it mean to be purebred?
A purebred organism is an organism whose
immediate ancestors are genetically uniform.
In genetics, the mating of two organisms is called a
cross.
What is the ratio of males to females in this
room right now including the instructors?
If I have 3 red marbles and 5 green
marbles, what is the ratio of green
marbles to red marbles?
If I have 3 children with brown eyes, and 1
child with blue eyes, what is the FRACTION
of my children with blue eyes?
The Law of Segregation (Mendel’s First Law).
1. Organisms inherit TWO copies of each gene; one
from each parent.
2. Organisms donate only ONE copy of each gene to
their gametes (sperm or egg).
This means the TWO copies must segregate during
formation of sperm or egg.
Traits, Genes, and
Alleles
6.4, 6.5
EQ: What is the relationship
between traits and genes?
A gene is a section of DNA that is coded to produce a
specific protein.
An allele is a form of gene located at a specific
location (locus) on a homologous chromosome.
Your cells have 2 alleles for each gene. Both from
mom and dad.
Locus refers to the specific location on homologous
chromosomes.
If the two alleles are the same and are located at the
same place on each chromosome, they are called
homozygous.
Recall that homo- means “same”.
If the two alleles at the same locus are different, they
are called heterozygous.
Recall that hetero- means “different”.
A dominant allele is expressed even if only 1 copy of
THAT allele is present. In this case, it would be B not
b. This is why most of the offspring would have brown
eyes.
A recessive allele is expressed only if both copies of
THAT allele are present. In this case, it would be bb
not B. This is why only 1 in 4 offspring have blue eyes.
This baby has brown eyes, but it’s genotype (in this
case) could be BB or…it could be Bb.
The genotype includes both sets of genes for a trait,
including the gene the isn’t expressed.
This girl’s genotype MUST be bb and not BB or Bb
because she has blue eyes.
This girl’s phenotype is blue eye color because it is the
color being expressed.
Phenotype is the physical characteristic being
expressed regardless of the genotype.