Download Genetics and Inheritance

Genetics and Inheritance ©
by Dr H – W Winter
Pukekohe High School NZ
Cells & their Nuclei
• All living things are made up by “cells”, the
building blocks of tissue.
• Most cells have a “nucleus”, the place
where the plans (or “blue-prints”) for
making the cell (and the rest of the whole
organism) are kept.
Chromosomes
• The plans for your body (these blue-prints) are
called chromosomes.
• They are pairs of strands of thread-like, very
long molecules made of a
material called DNA.
• One of the strands in a
pair came from the male,
the other from the
female sex cell,
A pair of
when the organism
chromosomes
was conceived.
made from DNA
Magnifying Details
nucleus
nucleus
cell
one gene
chromosome
inherited from
mother
chromosome
inherited from
father
chromosomes
in the nucleus
a pair of
chromosomes
made from DNA
Magnifying Details
2
2
1
5
6
7
3
4
Genes
• On the chromosomes are many tiny units
called genes (many thousands).
• One gene is responsible for one genetic
feature.
(e.g. tongue rolling or left-handedness etc)
R
r
Alleles, dominant & recessive
•
If the genetic information from one parent says something different from the
other parent, we have 2 different alleles. In our example mother’s allele
says: “right handed” and father’s allele says “left-handed”.
•
The “more influential one” of the 2 alleles will win and will be used; it is
called the dominant allele; it is given a capital letter (R).
•
The allele that loses the competition is called the recessive allele;
it is given a lower case letter (r).
•
In our example right-handedness is dominant over left-handedness, the
child is right-handed (but is also a carrier of the allele for left-handedness).
R
r
Fertilisation at Chromosomal Level
A sperm contains 50% of
the chromosomes (in blue)
Fertilisation:
The sperm penetrates the
egg and its chromosomes
enter – male and female An egg contains the other
chromosomes pair up, the 50% of the chromosomes
(in red)
egg becomes a zygote
Mitosis
identical
copying
• Mitosis is the process of making identical
copies of cells
• …. it is used to repair damaged tissue
• …. and when an organism grows
Meiosis
• Is a 2 stage cell division that makes 4 gametes with half the
chromosomes from one normal cell.
• In this process the alleles from the 2 strands of chromosomes are
mixed up to provide the basis for variation.
1st division after the
mother’s and father’s
chromosomes were
mixed up randomly
2nd division producing
sex cells that are
only having half of
the chromosomes
Discontinuous Variation
(aka either/or variation or discrete variation)
• You either have it or you don’t.
• Examples: left-handedness, gender,
tongue rolling, widow’s peak, curly hair
etc.
• Usually expressed as a % value (e.g. 25%
of the children of this family cannot roll
their tongue).
• Usually dependent on one gene that has
one dominant and one recessive allele.
Continuous Variation
• A wide range of values is possible
• Examples: height, shoe size, hair colour, hand span etc.
• A “bell-shaped” curve shows the distribution
Number
of Y 10
students
1.60 m
Average
height
Height of
Y 10
students
Georg Mendel
• Austrian monk
• born in 1822
• in monastery known for
research and teaching
• after his death (1884) acknowledgment of his
discoveries in 1900
Experiments with Pea Plants
- Seed coat colour (gray or white)
- Seed shape (round or wrinkled)
- Seed colour (yellow or green)
- Pod colour (green or yellow)
- Flower position (axial or
terminal)
- Pod shape (inflated or
constricted)
- Stem length (tall or dwarf)
Definitions you must know:
• Homozygous or “true breeding“
means that an organism (like a pea plant)
has only one type of alleles in its genetic
make-up
eg: GG or gg
• Heterozygous is the opposite to
homozygous, 2 different alleles are found
in the gene eg: Gg or gG
Definitions you must know:
• Genotype
describes the genetic make-up of an organism,
the allele combinations
eg: Gg or gg or GG, heterozygous or
homozygous
• Phenotype
describes the appearance of an organism, the
looks or type (tongue roller, left-hander, green
seed pod, white flower etc)
Punnet Squares
• are systematic diagrams to show the
statistical probability of either/or variation in
the offspring.
Parent 1 (genotype GG)
makes gametes
G
Parent 2 (genotype gg)
makes gametes
g
g
G
The thing that puzzled Mendel
Cross-Pollination of Pure-breed Plants
- cross-pollination
between homozygous
(true breeding) green
and yellow pods
- all F1 green
F1 Generation
Gg = heterozygous
F2 Generation
- self-pollination of
green F1 plants
- ¾ in F2 green,
¼ yellow
- 3 : 1 ratio in pod
colour in F2
G = dominant = green
g = recessive = yellow
GG, gg = homozygous