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Transcript 4.1,_4.2C_Traits

Chapter 4.1, 4.2C
Make a list of the characteristics you
see in the 3 children in the picture:
4.1: Parents and Offspring are Similar
 Traits are either:
 Inherited: traits you get from
your parents (hair color, eye color,
face shape, etc…)
 Acquired: developed over your
lifetime (learned behaviors, i.e. reading,
writing, riding a bike, etc…)
Traits are Controlled by Genes
 Genes: located on chromosomes and code
for a particular product (trait)
 You inherit genes from your parents
 Your cells contain 23 chromosome pairs
(homologs) to equal your 46 chromosomes
You received half of
You received half of
each homolog from
each homolog from
your mother.
your father.
Traits are Controlled by Genes
 Each homolog contains sites where genes
are located.
 Though the gene may be present on both,
the form of the gene may be different.
 Alleles: the various forms of the same gene
Chromosome Pairs
 Humans chromosomes are numbered 1-
22; the 23rd pair are the sex chromosomes
 Sex chromosomes are X-chromosomes
and Y-chromosomes
 Female: XX
 Male: XY
Gregor Mendel made some important
discoveries about heredity…
 Performed an experiment with pea plants
 Discovered that each plant must have two
factors for each possible trait, one factor
from each parent (Mendel’s “factors” are
now known as genes and alleles)
 Alleles interact to produce traits
Alleles Interact to Produce Traits
 Phenotype: describes your physical
characteristics that can be observed (eye
color, hair color)
 Genotype describes the actual
genes that you have on your
DNA; not always obvious
Alleles Interact to Produce Traits
 Mendel found that we have two copies of
each allele (one from mom, one from dad)
 Alleles can be
 Dominant
 Physically expressed regardless of what other allele it
is paired with
 Always expressed as a capital letter (T) (Ex: tallness
is the dominant trait for pea plant height)
 Recessive
 Physically expressed only when paired with another
recessive allele
 Always expressed as a lower-case letter (t) (Ex:
dwarfism is the recessive trait for pea plant height)
4.2: Punnett Squares
 Punnett Squares show possible outcomes for
 Mendel noticed that traits are inherited in
 Punnett squares illustrate how the parents’
alleles might combine in offspring.
 Each parent has two alleles for a particular
gene. An offspring receives one allele from
each parent. A Punnett square shows how the
parents’ alleles may be passed on to potential
Punnett Squares
 The letter “T”/”t” will refer to height (“T”
being talldominant; “t” being
Ratios and percentages can express
the probability of outcomes
 Probability: the likelihood or chance of a
specific outcome in relation to the total
number of possible outcomes
 Ratio: compares the relationship of two
 Percentage: a ratio that compares a
number to 100
 A TT (tall) plant is crossed with a tt (short
 What is the ratio of tall to short offspring?
 What percentage of the offspring will be tall?
 It is important to realize that Punnett
squares and probability do not guarantee
the outcome of a genetic cross. They
indicate the probability of different
outcomes. Actual experimental results
may not match predicted outcomes.