Download Ch. 10 – Sexual Reproduction and Genetics Meiosis provides

Ch. 10 – Sexual Reproduction and Genetics
Meiosis provides variation!
Meiosis – type of cell division that produces haploid cells
(gametes).
 Meiosis produces cells that reduces the number of
chromosomes by half.
 Meiosis occurs in the reproductive structures of
organisms that reproduce sexually.
 haploid cells (n) – have half the number of
chromosomes
 diploid cells (2n) – formed after fertilization.
 Meiosis produces 4 daughter cells that are not
identical (How does this compare with mitosis?)
 Meiosis involves two consecutive cell divisions
 Genetic Recombination – 2 ways meiosis gives us
variability in gametes:
1. Independent assortment – genes for traits may come
from either parent due to random assortment during
meiosis.
2. Crossing over- homologous chromosomes and their
sister chromatids are so closely bound together that an
exchange of genetic material takes place. See diagram pg.
272. This results in new combinations of alleles on chromosomes.
Humans have 46 chromosomes (or 23 pairs) numbered 1 – 23.
 Autosomes – chromosomes #1 – 22
 Sex Chromosomes – 23rd pair of chromosomes in humans
determine the individuals sex. If 23rd pair of chromosomes is:
 XX = female (Females can only produce gametes with X
chromosomes)
 XY = male (Males can produce gametes with either X or Y
chromosomes)
Homologous chromosomes – contain DNA that codes
for the same genes.
 The organism gets one chromosome from each
parent.
 Homologous chromosomes are not identical!!
Problems that can occur during meiosis:
 polyploidy – more than the usual number of
chromosome sets. Rare in animals because it almost
always causes death. Frequent in plants causing larger
and healthier flowers and fruits.
 Wheat (6n), sugar (8n)
 Triploid (3n)
Asexual reproduction – the offspring inherits all of its
chromosomes from a single parent. Genetically identical to
parent (i.e. bacteria, plants can reproduce asexually and
sexually)
Heredity – the passing of characteristics from parents to
offspring
Traits – characteristics that are inherited
Genes – segments on DNA that control production of
proteins which determine characteristics of an individual.
Genetics – the study of heredity
Gametes – sex cells (sperm and egg). These cells are
haploid (n) produced by meiosis.
Zygote – fertilized cell
Fertilization – process of combining n cells from female
parent with n cells from male parent to produce 2n offspring.
What about HUMANS?
 Genetic variation in humans:
 One human (with 23 chromosomes from each
parent) can produce 8 million different gametes, or
70 trillion different zygotes!
Gene linkage – genes that are located close to each other
on a chromosome are said to be linked and usually travel
together during gamete formation.
Mendelian Genetics
Gregor Mendel – Austrian monk who cross-pollinated pea
plants to study genetics. He crossed tall pea plants with
short pea plants. The result of these crosses is called a
hybrid.
Mendel concluded that each organism has two factors that
control each of its traits. We now know these are called
genes and are located on chromosomes. These different
forms for the same gene are called alleles.
Allele – The alternative form of a single gene that can be
passed down. Most genes have 2 alleles – a dominant and a
recessive. For example a red flowered plant (red is
dominant) may have a red allele and a white allele.
The Rule of Dominance
 In the F1 generation only one trait was observed – this
is the dominant trait. The trait that was not observed in
the F1 generation is the recessive trait.
 The dominant trait is shown with a capital letter (T for
tall). The recessive trait is shown with a lower case
letter (t for short). The same letter is used for the same
trait.
P1 – “parent”
generation
F1 “filial” generation
results from
crossing plants
in P1 generation.
F2 – second filial
generation –
results from
crossing plants
in F1 generation.
The Law of Segregation
 The two alleles for each trait separate during meiosis
 During fertilization the two alleles for that trait unite
Phenotype – the way an organism looks and behaves (tall).
Genotype – the allele combination that an organism
contains (TT or Tt)
Homozygous – when an organism’s alleles are the same
(TT or tt). Short plants are always homozygous.
Heterozygous – when an organism has two different alleles
(Tt). Also known as a hybrid.
1905 – Reginald Punnett (English biologist) came up with a
shorthand way to represent the expected proportions of
possible genotypes in offspring – the Punnett Square.
At first, Mendel performed many different crosses for several
different traits but he only tested one trait at a time:
Dihybrid cross - crosses of plants with two different traits.
Mendel concluded that genes for different traits are inherited
independent of each other, this is known as the
Law of Independent Assortment:
 Random distribution of alleles during gamete formation
Punnett Square for a dihybrid cross:
Do guinea pig worksheet example together.
 Long fur is dominant
o LL or Ll = long hair (dominant)
o ll = short hair (recessive)
 Black fur is dominant
o BB or Bb = black (dominant)
o bb = white (recessive)
Third Problem: BbLl X bbLl
 1st: Make 4 possible gametes for each parent:
BbLl:
bbLl:
 2nd: Draw a table with 16 squares
 3rd: Put gametes along the top and side of square
 4th: Fill in squares