Genetics
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Genetics Similarity and variation
1
The scientific study of heredity began More than a century ago with the Work of an Austrian monk named Gregor Mendel.
The passing of traits from parents To offspring is called heredity.
2
Mendel’s initial experiments were Monohybrid crosses. A monohybrid cross is a cross that Involves 1 pair of contrasting traits. For example, crossing a purple flower With a white flower plant to see What happens.
3
Mendel’s pea experiments focused On 7 traits that could be Easily viewed and studied. Flower color, seed color, Seed shape, pod color, pod shape, Flower position, and plant height.
4
5
Mendel carried out his experiments In 3 steps… 1) Mendel allowed each variety of Garden pea to self-pollinate for Several generations. This method ensured that each Variety was true-breeding for a Particular trait. All the offspring Would display only 1 form of a Particular trait. These true breeding plants served As the parental generation in Mendel’s Experiments. The parental generation (P 6
2) Mendel then cross pollinated 2 P Generation plants that had contrasting Forms of a trait. Mendel called the offspring of the P generation the 1st filial generation, Or the F1 generation. He then examined each F1 plant And recorded the number of F1 Plants expressing each trait. 3) Finally Mendel allowed the F1 Generation to self pollinate. He Called the offspring of the F1 Generation the 2nd filial generation 7
8
9
Mendel’s results did not support the Blending hypothesis. Mendel correctly concluded that each Pea has 2 separate “heritable factors” For each trait, 1 from each parent. Mendel came up with 4 hypotheses That were directly based on The results of his experiments. 10
1) For each inherited trait, an Individual has 2 copies of the gene; One from each parent. 2) There are alternative versions Of genes. Today the different Versions of a gene are called Alleles. Each allele can be passed On when the individual reproduces.
11
12
3) When two different alleles occur
together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. Mendel described the expressed form of the trait as dominant. The trait that was not expressed when the dominant form of the trait was present was described as recessive. 13
4) When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes one allele.
14
•Homozygous - If the two alleles of a particular gene present in an individual are the same. •Heterozygous - If the alleles of a particular gene present in an individual are different. •Dominant alleles are indicated by writing the first letter of the trait as a capital letter. •Recessive alleles are also indicated by writing the first letter of the dominant trait, but the letter is lowercase. •In heterozygous individuals, only the dominant allele is expressed; the recessive allele is present but unexpressed. •Genotype -The set of alleles that an individual 15
The Laws of Heredity Mendel’s 1st Law - The Law of Segregation The first law, the law of segregation, states that the two alleles for a trait segregate (separate) when gametes are formed. The first law of heredity describes the behavior of chromosomes during meiosis At this time, homologous chromosomes and then chromatids are separated.
Mendel’s 2nd Law – Law of Independent Assortment Mendel found that for the traits he studied, the inheritance of one trait did not influence the inheritance of any other trait. The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation. 16
One simple way of predicting the Expressed results (not necessarily The actual results) of the genotypes Or phenotypes in a cross is to Use a Punnett square.
A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross. The possible gametes that one parent can produce are written along the top of the square. The possible gametes that the other parent can produce are written along the left side of the square. Each box inside the square is filled in with two letters obtained by combining the allele along the top of the box with the allele along the side of the box. Punnett squares can be used to predict the outcome of a monohybrid cross (a cross that considers one pair of contrasting traits between two individuals). Punnett squares allow direct and simple predictions to be made about the outcomes of genetic crosses. 17
18
19
Variations on Mendel’s Laws 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Pleiotropy 5. Epistasis 6. Polygenic Inheritance 7. Environmental Effects
20
Red CRCR
White CW CW
Parent generation
21
P Generation Red CRCR
White CW CW
Gametes CR
CW
Pink CRCW
F1 Generation
Gametes 1
/2
1
F2 Generation
CR
W /2 C
1
Sperm /2 CR
/ 2 CW
1
/ 2 CR
1
CR
Eggs /2 CW CRCR
1
CR CW 22
Allele IA IB
Carbohydrate A B
i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype
Red blood cell appearance
IAIA or IA i IBIB or IB i
Phenotype (blood group) A
B
IA IB
AB
ii
O
(b) Blood group genotypes and phenotypes
23
Pleiotropy
5,555×
Sickle cells
Clumping of cells and clogging of small blood vessels
Breakdown of red blood cells
Physical weakness
Impaired mental function
Anemia
Heart failure
Paralysis
Pain and fever
Pneumonia and other infections
Accumulation of sickled cells in spleen
Brain damage
Damage to other organs
Rheumatism
Spleen damage
Kidney failure 24
Epistasis
25
Polygenic Inheritance
26
Environmental Effects
27
Pedigree Scientists cannot control matings in humans So, they must gather as much information about a family history They organize it onto a family tree called a pedigree
28
29
Inheritance of Traits Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations. Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder. Scientists can determine several pieces of genetic information from a pedigree
30
Autosomal or Sex-Linked? If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is usually seen only in males. A sex-linked trait is a trait whose allele is located on the X chromosome. Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait. Scientists can determine several pieces of genetic information from a pedigree: Heterozygous or Homozygous? If individuals with autosomal traits are homozygous dominant or heterozygous, their phenotype will show the dominant characteristic. If individuals are homozygous recessive, their phenotype will show the recessive characteristic.
31
Complex patterns of heredity Most traits are not controlled by Simple dominant-recessive alleles.
Characters influenced by several genes When several genes influence a trait, the trait is said to be a polygenic trait. The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes. Familiar examples of polygenic traits in humans include eye color, height, weight, and hair and skin color.
32
Intermediate characters In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance For example, when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced.
33
34
Characters controlled by genes with 3 or more alleles Genes with three or more alleles are said to have multiple alleles Even for traits controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene. Characters with two forms displayed at the same time: For some traits, two dominant alleles are expressed at the same time. In this case, both forms of the trait are displayed, a phenomenon called codominance. Codominance is different from incomplete dominance because both traits are displayed.
35
Characters influenced by environment An individual’s phenotype often depends on conditions in the environment Because identical twins have identical genes, they are often used to study environmental influences Because identical twins are genetically identical, any differences between them are attributed to environmental influences
36
1
The scientific study of heredity began More than a century ago with the Work of an Austrian monk named Gregor Mendel.
The passing of traits from parents To offspring is called heredity.
2
Mendel’s initial experiments were Monohybrid crosses. A monohybrid cross is a cross that Involves 1 pair of contrasting traits. For example, crossing a purple flower With a white flower plant to see What happens.
3
Mendel’s pea experiments focused On 7 traits that could be Easily viewed and studied. Flower color, seed color, Seed shape, pod color, pod shape, Flower position, and plant height.
4
5
Mendel carried out his experiments In 3 steps… 1) Mendel allowed each variety of Garden pea to self-pollinate for Several generations. This method ensured that each Variety was true-breeding for a Particular trait. All the offspring Would display only 1 form of a Particular trait. These true breeding plants served As the parental generation in Mendel’s Experiments. The parental generation (P 6
2) Mendel then cross pollinated 2 P Generation plants that had contrasting Forms of a trait. Mendel called the offspring of the P generation the 1st filial generation, Or the F1 generation. He then examined each F1 plant And recorded the number of F1 Plants expressing each trait. 3) Finally Mendel allowed the F1 Generation to self pollinate. He Called the offspring of the F1 Generation the 2nd filial generation 7
8
9
Mendel’s results did not support the Blending hypothesis. Mendel correctly concluded that each Pea has 2 separate “heritable factors” For each trait, 1 from each parent. Mendel came up with 4 hypotheses That were directly based on The results of his experiments. 10
1) For each inherited trait, an Individual has 2 copies of the gene; One from each parent. 2) There are alternative versions Of genes. Today the different Versions of a gene are called Alleles. Each allele can be passed On when the individual reproduces.
11
12
3) When two different alleles occur
together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. Mendel described the expressed form of the trait as dominant. The trait that was not expressed when the dominant form of the trait was present was described as recessive. 13
4) When gametes are formed, the alleles for each gene in an individual separate independently of one another. Thus, gametes carry only one allele for each inherited trait. When gametes unite during fertilization, each gamete contributes one allele.
14
•Homozygous - If the two alleles of a particular gene present in an individual are the same. •Heterozygous - If the alleles of a particular gene present in an individual are different. •Dominant alleles are indicated by writing the first letter of the trait as a capital letter. •Recessive alleles are also indicated by writing the first letter of the dominant trait, but the letter is lowercase. •In heterozygous individuals, only the dominant allele is expressed; the recessive allele is present but unexpressed. •Genotype -The set of alleles that an individual 15
The Laws of Heredity Mendel’s 1st Law - The Law of Segregation The first law, the law of segregation, states that the two alleles for a trait segregate (separate) when gametes are formed. The first law of heredity describes the behavior of chromosomes during meiosis At this time, homologous chromosomes and then chromatids are separated.
Mendel’s 2nd Law – Law of Independent Assortment Mendel found that for the traits he studied, the inheritance of one trait did not influence the inheritance of any other trait. The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation. 16
One simple way of predicting the Expressed results (not necessarily The actual results) of the genotypes Or phenotypes in a cross is to Use a Punnett square.
A Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross. The possible gametes that one parent can produce are written along the top of the square. The possible gametes that the other parent can produce are written along the left side of the square. Each box inside the square is filled in with two letters obtained by combining the allele along the top of the box with the allele along the side of the box. Punnett squares can be used to predict the outcome of a monohybrid cross (a cross that considers one pair of contrasting traits between two individuals). Punnett squares allow direct and simple predictions to be made about the outcomes of genetic crosses. 17
18
19
Variations on Mendel’s Laws 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Pleiotropy 5. Epistasis 6. Polygenic Inheritance 7. Environmental Effects
20
Red CRCR
White CW CW
Parent generation
21
P Generation Red CRCR
White CW CW
Gametes CR
CW
Pink CRCW
F1 Generation
Gametes 1
/2
1
F2 Generation
CR
W /2 C
1
Sperm /2 CR
/ 2 CW
1
/ 2 CR
1
CR
Eggs /2 CW CRCR
1
CR CW 22
Allele IA IB
Carbohydrate A B
i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype
Red blood cell appearance
IAIA or IA i IBIB or IB i
Phenotype (blood group) A
B
IA IB
AB
ii
O
(b) Blood group genotypes and phenotypes
23
Pleiotropy
5,555×
Sickle cells
Clumping of cells and clogging of small blood vessels
Breakdown of red blood cells
Physical weakness
Impaired mental function
Anemia
Heart failure
Paralysis
Pain and fever
Pneumonia and other infections
Accumulation of sickled cells in spleen
Brain damage
Damage to other organs
Rheumatism
Spleen damage
Kidney failure 24
Epistasis
25
Polygenic Inheritance
26
Environmental Effects
27
Pedigree Scientists cannot control matings in humans So, they must gather as much information about a family history They organize it onto a family tree called a pedigree
28
29
Inheritance of Traits Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations. Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder. Scientists can determine several pieces of genetic information from a pedigree
30
Autosomal or Sex-Linked? If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is usually seen only in males. A sex-linked trait is a trait whose allele is located on the X chromosome. Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait. Scientists can determine several pieces of genetic information from a pedigree: Heterozygous or Homozygous? If individuals with autosomal traits are homozygous dominant or heterozygous, their phenotype will show the dominant characteristic. If individuals are homozygous recessive, their phenotype will show the recessive characteristic.
31
Complex patterns of heredity Most traits are not controlled by Simple dominant-recessive alleles.
Characters influenced by several genes When several genes influence a trait, the trait is said to be a polygenic trait. The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes. Familiar examples of polygenic traits in humans include eye color, height, weight, and hair and skin color.
32
Intermediate characters In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance For example, when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced.
33
34
Characters controlled by genes with 3 or more alleles Genes with three or more alleles are said to have multiple alleles Even for traits controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene. Characters with two forms displayed at the same time: For some traits, two dominant alleles are expressed at the same time. In this case, both forms of the trait are displayed, a phenomenon called codominance. Codominance is different from incomplete dominance because both traits are displayed.
35
Characters influenced by environment An individual’s phenotype often depends on conditions in the environment Because identical twins have identical genes, they are often used to study environmental influences Because identical twins are genetically identical, any differences between them are attributed to environmental influences
36
