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Introduction to Genetic Crosses
RR / BIO 137
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What is genetics?

• Genetics is the study of HOW traits are passed
  from parents to offspring.
• Offspring show some traits of each parent
• These traits from parents are passed onto the
  offspring by sex cells

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Body Cell vs. Sex Cell
Body Cell Sex Cell
make up most of the tissues and organs in body sperm or egg cell only
Has 23 pairs of chromosomes (total46 chromosomes) Has 23 chromosomes
The chromosomes are paired up Only 1 of each chromosome
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Chromosomes
BODY CELL
SEX CELL
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Genes

• A gene is a small section of chromosome that
  determines a specific trait
• Genes are arranged on a chromosome
• Each chromosome carries specific genes

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Genes
Same Trait
Gene 1
Gene 2

• The genes on chromosomes in body cells are paired
• 2 genes represent a trait in each body cell

Pair of chromosomes
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Fertilization

• FERTILIZATION we get genes from each of our
  parents
• The only way traits are passed onto offspring

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Dominant Genes

• Genes that keep other genes from showing their
  traits
• EX plant size
• T is for the tall gene for a tree

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Recessive Genes

• The genes that do not show their traits when
  dominant genes are present

EX plant size t is the gene for a short tree
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Dominant vs. Recessive

• Pure Dominant an organism with 2 dominant genes
  for a trait
• Pure Recessive an organism with 2 recessive
  genes for a trait
• Heterozygous an organism with 1 dominant and 1
  recessive gene for a trait
• The dominant gene will overpower the recessive
  gene.
• The dominant gene will be expressed

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Example Palm Tree Height
Dominant Recessive
Gene is depicted with a capital letter Ex T Gene is depicted by a lower case letter Ex t
TT pure dominant tt pure recessive
Tt heterozygous Tt heterozygous
TALL SHORT
TT or Tt
tt
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Punnett Square
Mothers Genes

• A way to show which genes can combine when and
  egg and sperm join
• Letters are used in place of genes
• Large letter dominant gene (F)
• Small letter recessive gene (f)

Fathers Genes
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Punnett Square

• Large letter goes first in heterozygous organisms
  (Ff)
• Lets do a cross between a pure dominant male
  (sperm) for free earlobes (FF) and a female
  (egg) who is heterozygous for free earlobes (Ff)

F f
FF Ff
FF Ff
F F
F is dominant over f
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What are the expected results?
F f
FF Free Earlobes Ff Free Earlobes
Ff Free Earlobes ff Attached Earlobes
F f
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Expected vs. Observed Results
Expected Observed
Can be calculated Actually seen
Predicted Not Predicted
Genotype The combination of genes one has Phenotype the physical trait that you can see
What you expect to observe What you actually observed
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Dominant/Recessive

• One allele is dominant over the other (capable of
  masking the recessive allele)

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Problem Dominant/Recessive

• In pea plants, purple flowers (P) are dominant
  over white flowers (p) show the cross between two
  heterozygous plants.

P p
GENOTYPES

- PP (1) Pp (2) pp (1) - ratio 121
P p
PHENOTYPES
- purple (3) white (1) - ratio 31