Gregor Mendel, a monk, did extensive research into heredity—the transmission of characteristics from parents to offspring. Mendel studied 7 traits of garden pea plants [height, flower position along stem, pod color, pod appearance, seed texture, seed color, flower color]. Mendel planted the seeds of his first generation many times to study the results. Pollination occurs when pollen grains produced in the male reproductive part of the plant, the anthers, are transferred to the stigma (female) Self-pollination occurs when the plant whose source is the pollen is the same as the stigma.Cross-pollination occurs between different plants. In experiments, removing the anthers of a plant can prevent self-pollination. True-breeding plants are said to always give off the same trait when they self pollinate [this was the old fashioned word for homozygous Dominant and Recessive]. The P generation are the parents, and the F1 and F2 represent the children (F stands for filial). Mendel also hypothesized that traits were Dominant and Recessive. Mendel made a few laws about his early genetics. The Law of Segregation
Each reproductive cell, or gamete, receives one factor of each pair of the parent. [The factors are segregated] The Law of Independent Assortment
Factors spate independently of one another during the formation of gametes. [Just because something is dominant in one thing, for example, color, doesn’t mean its recessive for something else, like height]
Most of what Mendel discovered has been backed up today. Molecular Genetics is the study of the structure and function of chromosomes and genes. Because chromosomes occur in pairs, so do genes. Each of two or more forms of genes are called alleles. These are represented by capital letters for dominant traits, and lowercase for recessive.
9.2 Genetic Crosses
An organism’s genetic makeup is its genotype. It is represented by letters showing dominance and recessive alleles. [Example: Rr, RR, rr]
An organism’s physical appearance is its phenotype. [Example: Blue Flowers, Long Stem, Rough Skin]
When both alleles of a pair are the same, it is said to be homozygous [RR,rr]
When both alleles are different, the organism is considered to be heterozygous[Rr]. Probability is the likelihood that a specific event will occur. To find the probability, divide the amount of times something happened(or was expected to happen) by the amount that could happen. The results of probability become more apparent with more trials.
A cross in which only one characteristic is crossed is called a monohybrid cross. The offspring of this are called monohybrids. A Punnet square is a tool (figure 2) to estimate the outcome of a breeding. For example, if you cross something that is homozygous dominant, RR, with something homozygous recessive, rr, every child will have one capital R, or dominant trait, and therefore each will exhibit the dominant trait. The ratio of the genotypes that appear in offspring is called the genotypic ratio. The ratio of the offspring’s phenotypes is called the phynotypic ratio. genotypic ratio= Homozygous Dominant: Heterozygous: Homozygous Recessive
For example, if two heterozygous traits were crossed, the genotypic ratio would be: 1:2:1, because there is 1 RR, 2 Rr, 1 rr phenotypic ratio= Dominant: Recessive
For example, if two heterozygous traits were crossed, the phenotypic ratio would be: 3:1, because there is 1 RR, 2 Rr, 1 rr, and 3 of those exhibit the dominant trait.
A testcross can be performed to find out the genotype of an organism by crossing it with something that is homozygous recessive for the trait. This reveals whether or not something is Homozygous Dominant or Heterozygous.
Until now, all relationships have been complete dominance, when one dominant allele looks the same as 2 dominant alleles. Incomplete dominance is when a heterozygous organism shows a phenotype between Dominant and Recessive. In certain flowers, both the dominant (R) and recessive (r) alleles influence the phenotype. However, neither is completely dominant over the other. When red (RR) are crossed with white (rr), all flowers are pink. Crossing 2 heterozygous individuals would result in the same genotypic ratio and phenotypic ratio 1:2:1 [(1 RR: 2 Rr: 1 rr) or (1 Red: 2 Pink: 1 White)] Codominance is when there are more than one trait, but neither is really dominant over the other; they coexist. An example is blood type.
A Dihybrid cross is a cross in which two characteristics are crossed. The offspring are called dihybrids. The punnet squares are set up similarly, however, it is now a 4 x 4 array. Each column/row is labeled by two letters instead of one. The two letters indicate a possible gamete. [For example, if one parent is rryy, the only gamete possible is ry. If a parent is RrYy, the possible gametes are RY, Ry, rY, ry.]
In a heterozygous dihybrid cross, the results are always the following:
9/16 Dominant Phenotype [RRYY, RRYy, RrYY, RrYy]
3/16 Dominant First, Recessive Second [RRyy, Rryy]
3/16 Recessive First, Dominant Second [rrYY, rrYy]
1/16 Recessive Phenotype [rryy]
Figure 2
Figure 3
Chapter 12: Inheritance Patterns
12.2 Human Genetics
In the early 1900s, Thomas Hunt Morgan began experimenting with the fruit fly. He noticed that chromosomes were the same in male and female flies, except one pair. Females had two X’s, while males had XY. The X and Y chromosomes are called sex chromosomes and contain genes that determine the gender of an individual. Humans have 22 pairs of autosomes and 1 pair of sex chromosomes. All egg cells are X. Sperm cells have an equal chance of getting an X or a Y from the male. In a male mammal the Y chromosome contains a gene called SRY (Sex-determining Region Y). This develops into the testes if the child is male.
When Morgan crossed homozygous flies, in the F2 generation, he found the expected ratio of 3 red eyed flies to one. However, all the white eyed flies were male. Basically, Morgan realized that the trait was linked to the X chromosome. There are many more X linked traits than Y linked because the X chromosome is larger. Linked genes are pairs of genes that tend to be inherited together. The further apart two genes are located on a chromosome, the more likely a cross-over will occur. A chromosome map is a diagram that shows the linear order of genes on a chromosome. Each map unit is a frequency of crossing over 1 percent.
A mutation is a change in the DNA code. Germ-cell mutations occur in an organisms gametes. They do not affect an organism directly, but can be passed on. Somatic cell mutations take place in the body cells and affect the organism. Examples are cancer and cystic fibrosis. Lethel mutations cause death, usually before birth. There are 3 types of chromosome mutations. In deletion a letter is simply left out. In an inversion, a portion of code flips around. In a Translocation, a piece breaks off and attaches to another chromosome. In nondisjunction, a chromosome fails to separate from its homologue during meiosis. This can cause Down Syndrome. In Substitution, one nucleotide is switched for another. Frameshift is when one change changes the groupings of 3’s following it, whether by addition or deletion. This is usually very harmful.
12.2 Human Genetics
A pedigree is a diagram that shows how a trait is inherited over several generation. Pedigrees can help determine patterns of inheritance, which are used to make judgement about whether something is sex linked or not. These diagrams can also show if someone is a carrier, not affected by the trait but able to pass it on.
Some traits are polygenic, like skin color, and are the result of many different genes. Many human characteristics are complex characters; they are influenced by genes and environment. Genes with more than 3 alleles are said to have multiple alleles. They are usually all codominant. [an example of this would be blood types, A, B, O, AB] Humans also display incomplete dominance. [example, straight hair, curly hair, wavy hair]Sex –influenced traits are exhibited when a male and female have the same genotype for something yet different phenotypes. Amniocentesis and Chorionic villi sampling are both viable methods of determining whether a baby has a genetic disorder. Genetic counseling can be given to help a family understand their history of genetic diseases. Gene therapy is a technique that places a healthy copy of a gene in a body whose copy is defective.
9.1 Mendel's Legacy
Gregor Mendel, a monk, did extensive research into heredity—the transmission of characteristics from parents to offspring. Mendel studied 7 traits of garden pea plants [height, flower position along stem, pod color, pod appearance, seed texture, seed color, flower color]. Mendel planted the seeds of his first generation many times to study the results. Pollination occurs when pollen grains produced in the male reproductive part of the plant, the anthers, are transferred to the stigma (female) Self-pollination occurs when the plant whose source is the pollen is the same as the stigma. Cross-pollination occurs between different plants. In experiments, removing the anthers of a plant can prevent self-pollination. True-breeding plants are said to always give off the same trait when they self pollinate [this was the old fashioned word for homozygous Dominant and Recessive]. The P generation are the parents, and the F1 and F2 represent the children (F stands for filial). Mendel also hypothesized that traits were Dominant and Recessive. Mendel made a few laws about his early genetics.
The Law of Segregation
Each reproductive cell, or gamete, receives one factor of each pair of the parent. [The factors are segregated]
The Law of Independent Assortment
Factors spate independently of one another during the formation of gametes. [Just because something is dominant in one thing, for example, color, doesn’t mean its recessive for something else, like height]
Most of what Mendel discovered has been backed up today. Molecular Genetics is the study of the structure and function of chromosomes and genes. Because chromosomes occur in pairs, so do genes. Each of two or more forms of genes are called alleles. These are represented by capital letters for dominant traits, and lowercase for recessive.
9.2 Genetic Crosses
An organism’s genetic makeup is its genotype. It is represented by letters showing dominance and recessive alleles. [Example: Rr, RR, rr]
An organism’s physical appearance is its phenotype. [Example: Blue Flowers, Long Stem, Rough Skin]
When both alleles of a pair are the same, it is said to be homozygous [RR,rr]
When both alleles are different, the organism is considered to be heterozygous[Rr].
Probability is the likelihood that a specific event will occur. To find the probability, divide the amount of times something happened(or was expected to happen) by the amount that could happen. The results of probability become more apparent with more trials.
A cross in which only one characteristic is crossed is called a monohybrid cross. The offspring of this are called monohybrids. A Punnet square is a tool (figure 2) to estimate the outcome of a breeding. For example, if you cross something that is homozygous dominant, RR, with something homozygous recessive, rr, every child will have one capital R, or dominant trait, and therefore each will exhibit the dominant trait. The ratio of the genotypes that appear in offspring is called the genotypic ratio. The ratio of the offspring’s phenotypes is called the phynotypic ratio.
genotypic ratio= Homozygous Dominant: Heterozygous: Homozygous Recessive
For example, if two heterozygous traits were crossed, the genotypic ratio would be: 1:2:1, because there is 1 RR, 2 Rr, 1 rr
phenotypic ratio= Dominant: Recessive
For example, if two heterozygous traits were crossed, the phenotypic ratio would be: 3:1, because there is 1 RR, 2 Rr, 1 rr, and 3 of those exhibit the dominant trait.
A testcross can be performed to find out the genotype of an organism by crossing it with something that is homozygous recessive for the trait. This reveals whether or not something is Homozygous Dominant or Heterozygous.
Until now, all relationships have been complete dominance, when one dominant allele looks the same as 2 dominant alleles. Incomplete dominance is when a heterozygous organism shows a phenotype between Dominant and Recessive. In certain flowers, both the dominant (R) and recessive (r) alleles influence the phenotype. However, neither is completely dominant over the other. When red (RR) are crossed with white (rr), all flowers are pink. Crossing 2 heterozygous individuals would result in the same genotypic ratio and phenotypic ratio 1:2:1 [(1 RR: 2 Rr: 1 rr) or (1 Red: 2 Pink: 1 White)] Codominance is when there are more than one trait, but neither is really dominant over the other; they coexist. An example is blood type.
A Dihybrid cross is a cross in which two characteristics are crossed. The offspring are called dihybrids. The punnet squares are set up similarly, however, it is now a 4 x 4 array. Each column/row is labeled by two letters instead of one. The two letters indicate a possible gamete. [For example, if one parent is rryy, the only gamete possible is ry. If a parent is RrYy, the possible gametes are RY, Ry, rY, ry.]
In a heterozygous dihybrid cross, the results are always the following:
9/16 Dominant Phenotype [RRYY, RRYy, RrYY, RrYy]
3/16 Dominant First, Recessive Second [RRyy, Rryy]
3/16 Recessive First, Dominant Second [rrYY, rrYy]
1/16 Recessive Phenotype [rryy]
Chapter 12: Inheritance Patterns
12.2 Human Genetics
In the early 1900s, Thomas Hunt Morgan began experimenting with the fruit fly. He noticed that chromosomes were the same in male and female flies, except one pair. Females had two X’s, while males had XY. The X and Y chromosomes are called sex chromosomes and contain genes that determine the gender of an individual. Humans have 22 pairs of autosomes and 1 pair of sex chromosomes. All egg cells are X. Sperm cells have an equal chance of getting an X or a Y from the male. In a male mammal the Y chromosome contains a gene called SRY (Sex-determining Region Y). This develops into the testes if the child is male.
When Morgan crossed homozygous flies, in the F2 generation, he found the expected ratio of 3 red eyed flies to one. However, all the white eyed flies were male. Basically, Morgan realized that the trait was linked to the X chromosome. There are many more X linked traits than Y linked because the X chromosome is larger.
Linked genes are pairs of genes that tend to be inherited together. The further apart two genes are located on a chromosome, the more likely a cross-over will occur. A chromosome map is a diagram that shows the linear order of genes on a chromosome. Each map unit is a frequency of crossing over 1 percent.
A mutation is a change in the DNA code. Germ-cell mutations occur in an organisms gametes. They do not affect an organism directly, but can be passed on. Somatic cell mutations take place in the body cells and affect the organism. Examples are cancer and cystic fibrosis. Lethel mutations cause death, usually before birth. There are 3 types of chromosome mutations. In deletion a letter is simply left out. In an inversion, a portion of code flips around. In a Translocation, a piece breaks off and attaches to another chromosome. In nondisjunction, a chromosome fails to separate from its homologue during meiosis. This can cause Down Syndrome. In Substitution, one nucleotide is switched for another. Frameshift is when one change changes the groupings of 3’s following it, whether by addition or deletion. This is usually very harmful.
12.2 Human Genetics
A pedigree is a diagram that shows how a trait is inherited over several generation. Pedigrees can help determine patterns of inheritance, which are used to make judgement about whether something is sex linked or not. These diagrams can also show if someone is a carrier, not affected by the trait but able to pass it on.
Some traits are polygenic, like skin color, and are the result of many different genes. Many human characteristics are complex characters; they are influenced by genes and environment. Genes with more than 3 alleles are said to have multiple alleles. They are usually all codominant. [an example of this would be blood types, A, B, O, AB] Humans also display incomplete dominance. [example, straight hair, curly hair, wavy hair] Sex –influenced traits are exhibited when a male and female have the same genotype for something yet different phenotypes.
Amniocentesis and Chorionic villi sampling are both viable methods of determining whether a baby has a genetic disorder. Genetic counseling can be given to help a family understand their history of genetic diseases. Gene therapy is a technique that places a healthy copy of a gene in a body whose copy is defective.