genetics
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GeneticsMendel’s Laws of Heredity
GeneticsMain Concepts…
a. Sexual reproduction involves the random distribution of genes.
b. We have thousands of genes.c. Genes determine your traits.d. Genes line up on your chromosomes.e. Chromosomes are in the nucleus of the cell.f. Chromosomes duplicate and divide in cell
division.g. Chromosomes are typically in pairs.h. A diploid cell has complete set of chromosomes.
GeneticsFrog: 13 pairs
Pea: 7 pairs
Apple: 17 pairs
Chimpanzee: 24 pairs
Dog: 39 pairs
1. HEREDITY – the passing of characteristics from parents to offspring
2. Characteristics that are inherited are called TRAITS
Heredity
1. Gregor Mendel was an Austrian monk and was the first person to successfully study GENETICS, the branch of biology that studies heredity
Gregor Mendel
Pea Plants
1. Mendel chose to study the garden pea plant:
a) The male gamete is pollen and fuses with the female gamete in a process called FERTILIZATION
i. Fertilized egg = zygote
b) The transfer of male pollen to the female reproductive organ is called POLLINATION
1. Pea plants can reproduce by SELF-POLLINATION because each plant has both male and female gametes
2. Mendel performed a process called CROSS-POLLINATION by dusting pollen from one plant to any other plant of his choosing. This allowed him to control which plants mate with which plants
Pea Plant (con’t)
Research
1. Mendel was a careful researcher:
a) He studied only one TRAIT at a time (e.g. Height) and analyzed the data mathematically
b) Mendel used TRUE BREEDING plants for his crosses. This means the plants have the same traits for many generations (e.g. all purple flowers)
i. Pure bred
1. When Mendel crossed two different ALLELES for a trait he called this a HYBRID
2. When Mendel only crossed one trait at a time he called this a MONOHYBRID CROSS
Experiment
1. All pea plants are either tall (6ft) or short (2ft)
2. The 1st Generation:a) Mendel cross-pollinated a true breeding tall
pea plant with a true breeding short pea plant. The offspring were all tall pea plants
b) P1 Tall x short = All tall plants
Monohybrid Cross (Height)
Monohybrid Cross (Height)
1. The 2nd Generation:
a) Mendel allowed the tall offspring to self-pollinate.
b) F1 Tall x Tall = 75% Tall/25% Short
c) 3:1 ratio of tall
Mendel’s Experiments ReviewShort pea plants X Tall pea plantsQ: What did he get?A: All Tall pea plants!!!!
Then, Tall X Tall pea plantsQ: What did he get?A: 787 Tall pea plants & 277 short pea plants
3:1 Ratio = 75% tall : 25% short
Generations
1. The original (true breeding) parents are known as the P1 generation
a) P = “parent”
2. The offspring of the P1 generation are known as the F1 generation
a) F = “filial” (son or daughter)
3. Crossing two F1 plants creates the F2 generation
Conclusions from Mendel’s Experiments1. Factors = Genes2. Law of Dominance: in any pair of genes, one
may hide the appearance of the other.a) Dominant: expressedb) Recessive: masked, hidden, not expressed
3. Law of Segregation: During Meiosis chromosomes separate, and genes separate also.
4. Law of Independent Assortment: the pairs of genes (and chromosomes) separate randomly!
Rule of Dominance
1. Mendel also concluded that one allele is DOMINANT and one is RECESSIVE
2. The dominant allele is the one that shows up in the F1 generation (Tall)
a) Capital letter T
3. The recessive allele is the trait that is hidden in the F1 generation (Short)
a) Lowercase letter t
4. Capital letter is always written before the lowercase letterTt tT
1. A dominant trait (tall) is the result of either TT or Tta) TT = tall b) Tt = tall
2. A recessive trait (short) can ONLY be the result of tta) tt = short
3. The dominant allele always overrides the recessive allele, therefore, dominant traits are more common than recessive traits
Dominant/Recessive Traits
Purple is dominant White is recessive
Purple is present(White is hidden)
AA aa
Aa
AaAaAA aa
Alleles
1. Mendel concluded that each organism must have two factors that control each of its traits
2. We now know that traits are controlled by GENES and are located on chromosomes.
3. Genes exist in alternative forms called ALLELESa) Gene plant height
i. Alleles tall or shortii. Alleles dominant or recessive
1. Alleles are located in exactly the same position on homologous chromosomes
2. In a diploid organism, there are two alleles for a given gene
a) One from mother; one from father
3. A plant might have:a) 2 copies of the tall allele (TT)b) 2 copies of the short allele (tt)c) 1 tall and 1 short allele (Tt)
Alleles
♂
♀
Law of Segregation
1. The Law of Segregation – every individual has two alleles for each gene and each gamete receives one of these alleles
Law of Segregation
Each gamete only gets one allele
Each offspring has one allele from each parent
The Law of Independent Assortment
1. The Law of Independent Assortment – genes for different traits are inherited independently of each other
a) Height does not affect flower color; they are independent of each other
b) Independent assortment results in increased genetic diversity because of the shuffling of individual genes
Law of Independent Assortment
1. PHENOTYPE: physical appearance; what it looks like.
2. GENOTYPE: genetic make-up; combination of genes.
3. Represented by 2 letters.4. 3 possible genotypes.
EX. Green pea pod, pink flower, tall stems
EX. GG, Gg, gg
Genotype vs. Phenotype
Homozygous vs. Heterozygous1. Homozygous – two alleles are the SAME
a) Homozygous dominant TTb) Homozygous recessive ttc) True breeding (pure bred)
2. Heterozygous – two alleles are DIFFERENTa) Ttb) Hybrids
Traits due to 1 gene
1. Dominant2. Free earlobe3. Left handedness4. Tongue rolling5. Left arm on top6. Left thumb on top
1. Recessive2. Cleft chin3. Dimples4. Attached earlobe5. Second toe longer6. Widow’s peak7. Double jointedness
Human Genetics
Dimples
Cleft Chin
Widow’s Peak
Free and Attached Ear lobes
Second toe is longer than big toe
Tongue rolling
1. Reginald Punnett devised an easy system to predict genotypic outcomes of a cross called a PUNNETT SQUARE
2. Parents gametes are combined in every possible combination
3. Monohybrid cross – one gene (height)4. Dihybrid cross – two genes (height and color)
Punnett Squares
Father’s Gametes (sperm)
Moth
er’s
Gam
ete
s
(eg
g)
PossibleOffspring
#1
PossibleOffspring
#2
PossibleOffspring
#3
PossibleOffspring
#4
Monohybrid Cross
TT x ttSetting up the Punnett square
TT tt(Short)(Tall)
Mom Dad
T = tallt = short
T
T
x
t t
Solving the Punnett Square
T
t
T
tt
T
t
ttTT
T
(Tall) (Short)
Tall Tall
Tall Tall
(Heterozygous)
(Heterozygous) (Heterozygous)
(Heterozygous)
T = tallt = short
Mom Dad
TTt x TtSetting up the Punnett square
Tt t(Tall)(Tall)
TT
T
t
x
T t
Solving the Punnett Square
TT
t
t
t
TtTt
t
(Tall) (Tall)
Tall Tall
Tall Short
(Homozygous)
(Homozygous)(Heterozygous)
(Heterozygous)
1. Offspring outcomes can be expressed as:
a) Fractions – ¼ or ¾
b) Percent – 25% or 50% or 75%
c) Ratio – 3:1 or 1:2:1
Fractions, Percent, Ratios
Test Cross
1. A TEST CROSS is a cross of an individual of unknown genotype with an individual with a known genotype
a) Test crosses use homozygous recessive individuals because the phenotype always indicates the genotype (short = tt)
2. Example:a) Tall pea plant: genotype unknown (TT or Tt)b) Short pea plant: known genotype (tt)c) Cross the tall plant with the homozygous recessive and look at the
offspringi. If the offspring are all tall then the unknown genotype is TTii. If the offspring are 50% tall and 50% the unknown genotype is
Tt
Tall Plant(TT or Tt?)
Short Plant(tt)
P1
Test Cross
F1… TT TtOR
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