Classical Genetics. Gregor Mendel. The Work of Gregor Mendel. Genetics : the scientific study of heredity, or how traits are passed from one generation to the next Gregor Mendel’s Peas - Mendel was an Austrian monk who is credited as the “father of genetics.”. The Work of Gregor Mendel.
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Genetics: the scientific study of heredity, or how traits are passed from one generation to the next
Gregor Mendel’s Peas
- Mendel was an Austrian monk who is credited as the “father of genetics.”
- Mendel began working with pea plants that were true-breeding, meaning that if allowed to self pollinate, they would only produce offspring identical to themselves
-ex. Tall plants produce tall plants, green seeded plants produce green seeded plants
Genes and Dominance
- Mendel studied 7 different plant traits. Traits are characteristics such as height or flower color.
- Mendel called the original plants the P (parent) generation. The offspring were the F1 (first filial) generation .
- the offspring of crosses between parents of different traits are called hybrids
Mendel's F2 Generation
From his experiments, Mendel drew two conclusions
1) biological inheritance is determined by factors that are passed from one generation to the next. Today we call these factors genes.
- the different forms of a gene are called alleles
ex. gene (trait) is eye color
alleles (forms) are blue, brown, green, etc.
2) the second conclusion is called the principle of dominance
- the principle of dominance states that some alleles are dominant and others are recessive
- Mendel saw that some forms of traits would disappear during the F1 generation, but reappear during the F2 generation
- he explained this by showing that the alleles segregated during the formation of gametes, or sex cells.
Gametes are also known as
The offspring of crosses between parents with different traits are called
In a cross of a true-breeding tall pea plant with a true-breeding short pea plant, the F1 generation consists of
If a particular form of a trait is always present when the allele controlling it is present, then the allele must be
Genes and Probability
- the likelihood that an event will occur is called probability
ex. a coin landing on heads has a probability of 50%
- the principle of probability can be used to predict the outcome of genetic crosses
- the gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square
Organisms that have two identical alleles for a particular trait are called homozygous.
Organisms that have two different alleles for a particular trait are called heterozygous
The physical characteristics of an organism are called its phenotype
ex. tall plant
The genetic makeup of an organism is its genotype
ex. TT or Tt
Probability can be used to predict
Compared to 4 flips of a coin, 400 flips of the coin is
Organisms that have two different alleles for a particular trait are said to be
Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall?
- the principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes
ex. Seed color does not depend on seed shape
Beyond dominant and recessive alleles
- some alleles are neither dominant nor recessive , and many traits are controlled by multiple alleles or multiple genes
Incomplete dominance or Codominance
- the heterozygous phenotype is somewhere between the two homozygous phenotypes
ex. red flower + white flower = pink
Punnett square animation
- multiple alleles contribute to the phenotype ex. Blood type (i, IA, IB)
Type O = i i
Type A = IA IA or IA i
Type B = IB IB or IB i
Type AB = IA IB
- two or more genes control the outcome of a particular trait
ex. Skin color in humans is controlled by the combinations of 4 different genes
In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have
Mendel's principles apply to
A typical human body cell contains 46 chromosomes
- 44 autosomes and 2 sex chromosomes
Chromosomes are arranged in a karyotype, a picture of the chromosomes
Males and females differ in the sex chromosomes they contain
- males are XY
- females are XX
All egg cells carry a single X chromosome. However, half of all sperm cells carry a X chromosome while half carry a Y chromosome.
Scientists use a pedigree to help study how traits are passed from one generation to the next.
Pedigree: a chart which shows the relationships within a family
Square = Male
Circle = Female
carrier of the trait
not shaded = does not express the trait
shaded = express the trait
Since most human traits are polygenic (many genes), they can not be traced to a single parent
Many genes have been discovered through the study of genetic disorders
- they can be dominant or recessive
In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder
- caused by a recessive allele on chromosome 7
- causes digestive and respiratory problems
- only half of people with CF live into their 20s
Sickle Cell Disease
- causes hemoglobin to be less soluble
- changes the shape of red blood cells; become bent into the sickle-shape
Regions where malaria is common
Regions where the sickle cell allele is common
A chromosome that is not a sex chromosome is know as a(an)
An individual with a blood type phenotype of O can receive blood from an individual with the phenotype
Human genes and chromosomes
- on each chromosome there is the information for several hundred genes
ex. Gene for ALS is on 22nd chromosome
Sex-linked genes: genes located on the X or Y chromosome
ex. colorblindness, hemophilia, muscular dystrophy
Males have only 1 X chromosome and cannot be heterozygous for the trait. Females can be carriers (heterozygous).
The most common error occurs when homologous chromosomes fail to separate during meiosis. This is called nondisjunction.
Homologous chromosomes fail to separate
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.
- Down syndrome results from a nondisjunction of the 21st chromosome
- Turner’s syndrome (XO) and Klinefelter’s syndrome (XXY)