Transcript Chapter 6 - SchoolRack

Chapter 6
Heredity
Section 1: Mendel and His
Peas
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Humans share common characteristics
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But what makes you different than your
fellow classmates?
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i.e. feet, eyes, hands, etc.
Heredity – the passing of traits from
parents to offspring
How heredity works was the topic of
Gregor Mendel’s experiments
Who Was Gregor Mendel
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Mendel was born in 1822 in Austria
He grew up on a farm and learned about
cultivating flowers
After attending college, Mendel entered a
monastery, where he worked in the garden
He used the monasteries plants (garden
peas) to study how traits where passed on
from parents to offspring
Unraveling the Mystery
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Mendel’s choice of garden peas was a good
one for several reasons
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1) they grow quickly
2) they are self-pollinating
3) they come in many varieties
Self-pollinating plant – a plant that contains
both male and female reproductive structures
Peas Be My Ponder
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To keep things simple Mendel decided
to study only one trait at a time
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i.e. height or flower color
Pea plants have two forms for each
characteristic
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i.e. tall vs. short plants or purple vs. white
flowers
True –Breeding Plants
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Mendel was careful to use plants that were
true-breeding
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True-breeding plant – plants that self-pollinate
and always produce off-spring that have the same
trait as the parent plant
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Mendel decided to “cross” two plants with
different forms of the same trait
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i.e. tall parent plant produces 100% tall offspring
i.e. tall plant and short plant
He used a method called “cross-pollination”
Mendel’s First Experiment
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In his first experiment Mendel studied seven different
characteristics (we will focus on one – flower color)
Mendel crossed a true-breeding purple flowered plant
with a true breeding white flowered plant (parental
generation)
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All of the offspring had purple flowers!
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The offspring are known as the first generation(f1)
Mendel got similar results from all his different crosses
What happened to the white flowers?
One trait always appeared in the f1 generation, while
the other seemed to vanish
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Dominant trait – the trait that appeared
Recessive trait – the trait that seemed to vanish
Mendel’s Second Experiment
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Mendel allowed the f1 generation to self
pollinate
Do you know what happened?
The recessive trait (white flower)
showed up again
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The results were 75% purple to 25% white
This is a 3:1 ratio
A Brilliant Idea
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Mendel realized that his results could be
explained only if each plant had two sets of
instructions for each characteristic
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Each parent could only donate one set of
instructions to the offspring
Genes – the set of instructions
What do these instructions control?
Alleles – two genes that govern the same
characteristic
The Proof Is in the Punnett
Square
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Punnett Square – a tool used to
visualize all the possible combinations
of alleles
Lets do some examples on the board
Genotype – The inherited combination
of alleles
Phenotype – The organism’s outward
appearance
What Are the Chances?
It is important to understand that offspring
are equally likely to inherit either parents
allele
Even though there seems to be a 50/50
chance of inheriting either allele, it is more
similar to a coin toss and is completely random
Because of this the laws of probability must be
considered
Probability - is the mathematical chance
that an event will occur
Genotype Probability
The same method is used to calculate the
probability that an offspring will inherit a
certain genotype
For example in Mendel’s 2nd experiment
what is the probability that an offspring will inherit
two lowercase p’s (pp) and be white?
(1/2 x 1/2 = 1/4 = 25%)
Chapter 6 Quiz 1
1)_________ is the passing of traits from
parents to offspring
2) _________ is a plant that contains both male
and female reproductive structures
3) _________ the name Mendel gave to the trait
that seemed to disappear in his 1st experiment
4) _________ are two forms of the same gene
(one on moms chromosome and one on Dads)
5) __________ is a tool used to visualize all
possible combinations of inherited genes
Bonus) __________ carry the genes that
determine whether and organism is male or
female
Section 2: Meiosis
Two Kinds of Reproduction
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Asexual Reproduction – only one parent is needed
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1) the internal structures are copied (mitosis)
2) the cell then divides producing two identical cells
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Sexual Reproduction – two parent cells join together
to form a new individual
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Sex cells – the parent cells (sperm/eggs)
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contain half the normal amount of chromosomes
Homologous chromosomes – Identical chromosome pairs
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Most single-celled organisms reproduce this way as well as
most of the cells in your body
i.e. Humans have 23 pairs of chromosomes (46 total), 23 from
mom and 23 from dad
Why do sex cells have ½ the amount of
chromosomes?
Meiosis to the Rescue
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Walter Sutton discovered that genes are
located on chromosomes
Sex cells are made during meiosis
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Meiosis – produces new cells with ½ the usual
number of chromosomes
During meiosis the cells are copied once, but
go through two rounds of divisions producing
sperm and eggs with ½ the normal amount
of chromosomes
Male or Female
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Sex chromosomes – Chromosomes that
carry genes that determine whether an
offspring will be male or female
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In humans females have two X
chromosomes, while males have one X and
one Y
Chapter 6 Quiz 2
1.________ is the processes that produces
new cells with half the number of chromosomes
2.________ are pairs of chromosomes that
contain the similar genes for the same traits
3.________ is the phase of meiosis where
crossing over occurs
4.________ is the scientist who discovered that
genes where located on chromosomes
Bonus) ________ are flowers that always have
offspring identical to themselves