SEXUAL REPRODUCTION & GENETICS – CHP10
10.1 – Meiosis
10.2 – Mendelian Genetics
10.3 – Gene Linkage
Gregor Johann Mendel (1822–
1884) was a priest and scientist,
and is often called the father of
genetics for his study of the
inheritance of traits in pea plants
10.1 MEIOSIS
Objectives
1. Explain the reduction in
chromosome number that
occurs in Meiosis
2. Recognize and summarize the
stages of Meiosis
3. Analyze the importance of
meiosis in genetic variation
VOCAB
•Gene
•Homologous
chromosome
•Gamete
•Haploid
•Fertilization
•Diploid
•Meiosis
•Crossing over
Meiosis Summary
- division of gametes (sex cells)
- 2 nuclear divisions (meiosis I and meiosis II)
- results in FOUR cells from the parent
HAPLOID – cell with half the number of chromosomes (n) as a
diploid (2n)
DIPLOID – cell with two copies of each chromosome
Somatic (body cells) are diploid (2n) and gametes (sex cells)
are haploid (n)
If n = # of chromosomes
HUMAN sex cells = n = 23
Somatic cells = 2n – 46
Fertilization
- oocyte + sperm results in a diploid
zygote
Importance of Meiosis
- it results in GENETIC
VARIATION
HOW?
- Pairs of homologous
chromosomes line up at the
equator during METAPHASE I.
- Depending on how the
chromosomes line up, FOUR
gametes with FOUR different
combinations of chromosomes
can result.
-Genetic variation also is
produced by crossing over
(PROPHASE I) and during
fertilization when gametes
randomly combine.
Mendelian GENETICS – 10.2
Objectives
1. Explain the significance
of Mendel’s experiments
to the study of genetics
2. Summarize the law of
segregation and law of
independence assortment
3. Predict the possible
offspring from a cross
using a Punnett square
VOCABULARY
- allele
- dominant
- recessive
- homozygous
- heterozygous
- genotype
- phenotype
- hybrid
Q. Predict the possible offspring from a cross using a
Punnett square – VOCABULARY
P1 – parental generation
F1 or F2 – filial (offspring) generation
Allele – alternative form of a single gene
Homozygous – same alleles for single trait
Heterozygous – different alleles for single trait
Genotype
– organism’s GENes(alleles)
Phenotype – organism’s Physical/observable
characteristics
Hybrid - heterozygous
Purebred – homozygous
Test cross – involves breeding of an
organism that has the unknown
genotype with on that is
homozygous recessive for the
desired trait
Gene Linkage – occurs when a particular genetic allele for
genes are inherited jointly. They tend to stay together during
meiosis and are genetically linked.
While homologous pairs of chromosomes are independently assorted in
meiosis, the genes that they contain are also independently assorted
only if they are part of different chromosomes.
Genes in the same chromosome are passed on together as a unit. Such
genes are said to be linked. For example, the "A" and "B" alleles (in the
illustration below) will both be passed on together if the lower
chromosome is inherited. "A" and "B" are linked due to their occurrence
in the same chromosome. Similarly, "a" and "b" are linked in the other
chromosome.
Linked genes most likely account for such phenomena as red hair being
strongly associated with light complexioned skin among humans. If you
inherit one of these traits, you will most likely inherit the other.
Sex-linkage – When a trait is carried on the X or Y
chromosomes, it is called a sex-linked trait
- Don’t confuse this with
linked genes = when 2 genes are on the same chromosome
XY
Out of your 23 pairs of chromosomes
22 pairs = autosomes
1 pair = sex chromosome (XX or XY)
XX
Y
X
XX
X
XY
- You have to include the X
and Y chromosomes in the
Punnett Square
- Superscripts on the X
and Y denote which allele is
present
Red-Green Colorblindness
- Gene that controls this is on the X chromosome
Who is more likely to be color blind–
men or women?
Men: only 1 X chromosome –
if they have the recessive allele
they don’t have another X to
make up for it.
EXAMPLE: color
blindness
XX – mask the recessive
train as there are 2 X
XY – because there is only
1 X, they are affected by
the recessive trait
EXAMPLE: Hemophilia – recessive sex-linked
- The effects of this sex-linked, X
chromosome disorder are
manifested almost entirely in
males, although the gene for the
disorder is inherited from the
mother.
- Females have two X
chromosomes while males have
only one, lacking a 'back up' copy
for the defective gene.
- Females are therefore almost
exclusively carriers of the
disorder, and may have inherited
it from either their mother or
father.
QUEEN VICTORIA’S PEDIGREE