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Inheritance chapter 17 bio
Chromosomes, Genes and Proteins
Chromosomes: made of DNA, which contains genetic information in the form of genes
Gene: a length of DNA that codes for a protein
Allele: an alternative form of a gene
Inheritance of sex in humans is used with X and Y chromosomes.
Haploid nucleus: a nucleus containing a single set of unpaired chromosomes (e.g., sperm and egg)
Diploid nucleus: a nucleus containing two sets of chromosomes (e.g., in body cells)
The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.
Different sequences of amino acids give different shapes to protein molecules.
DNA and Protein Synthesis
DNA: controls cell function by controlling the production of proteins, including enzymes, membrane carriers, and receptors for neurotransmitters
DNA has 2 long strands and 4 nucleotides, AT and CG
mRNA has AU and CG bases
Overall, protein synthesis occurs outside of the nucleus in the cytoplasm
Protein synthesis has two stages:
Transcription (rewriting the base code of DNA into bases of mRNA)
Translation (using mRNA base sequence to build amino acids into a sequence in a protein)
How proteins are made:
the gene coding for the protein remains in the nucleus
messenger RNA (mRNA) is a copy of a gene
mRNA molecules are made in the nucleus and move to the cytoplasm
the mRNA passes through ribosomes
the ribosome assembles amino acids into protein molecules
the sequence determines the specific order of amino acids of bases in the mRNA
All body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs
Cell Division: Mitosis
Mitosis: The nuclear division gives rise to genetically identical cells
Mitosis is needed for:
Growth: in animals, each tissue provides its own new cells when needed.
Repair damaged tissues: for example, when you cut your skin, mitosis provides new cells to cover up cuts.
Replacement of worn-out cells
Asexual reproduction: in plants
The exact replication of chromosomes occurs before mitosis
During mitosis, the copies of chromosomes separate, maintaining the chromosome number in each daughter cell
Stem Cells: unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions
Cell Division: Meiosis
Meiosis: Reduction division in which the chromosome number is halved from diploid to haploid
Meiosis is involved in the production of gametes.
Meiosis results in genetic variation, so the cells produced are not all genetically identical.
Gametes such as sperm and ovum are produced via meiosis.
Monohybrid Inheritance
Inheritance: The transmission of genetic information from generation to generation.
Here are some common terminologies you should know to understand monohybrid inheritance.
Terminologies
Genotype: the genetic makeup of an organism in terms of the alleles present (e.g. Tt or GG)
Phenotype: the observable features of an organism (e.g. tall plant or green seed)
genotype + environment + random variation → phenotype
Homozygous: having two identical alleles of a particular gene (e.g. TT or gg). Two identical homozygous individuals that breed together will be pure-breeding
Heterozygous: having two different alleles of a particular gene (e.g. Tt or Gg), not pure-breeding
Dominant: an allele that is expressed if it is present (e.g. T or G)
Recessive: an allele that is only expressed when there is no dominant allele of the gene present (e.g. t or g)
Pedigree Diagram
Pedigree diagrams trace the inheritance pattern of a specific characteristic (usually a disease) through generations of a family.
Pure Breeding: The individual is homozygous for that characteristic
This can determine the probability that someone in the family will inherit the genetic disorder.
Genetic Diagrams
Monohybrid Inheritance can be determined using a genetic diagram known as a Punnett square.
A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring.
The dominant allele is shown using a capital letter, and the recessive allele uses the same letter but lowercase.
If you are asked to use your own letters to represent the alleles in a Punnett square, try to choose a letter that is obviously different as a capital than the lowercase so the examiner is not left doubt as to which is dominant and which is recessive.
1:1 Monohybrid Crosses
In this cross, there is a 1:1 ratio of boy to girl, meaning a 50% chance of the offspring being a boy and a 50% chance of the offspring being a girl.
3:1 Monohybrid Crosses
There is more variation in this cross, with a 3:1 ratio of brown eyes: blue eyes, meaning each offspring has a 75% chance of having brown eyes and a 25% chance of having blue eyes
Modification of Mendelian Genetics
Codominance
Codominance: a condition where two alleles of a gene are equally dominant (50%)
Both alleles are equally expressed in the phenotype of the heterozygote.
For example, if the parent phenotype is red and white, a co-dominant will have both red and white colour in the offspring.
ABO Blood Group (Codominance and Multiple Allele)
Inheritance of blood group is an example of codominance
There are three alleles for the blood group given by the symbols IA, IB and IO.
IA and IB are co-dominant giving blood group AB or IAIB, and both dominant to IO.
Sex-Linked Characteristics
Sex-linked characteristic: a characteristic in which the gene responsible is located on a sex chromosome, making it more common in one sex than in the other.
Generally, in the IGCSE syllabus, the most common is X-linked recessive trait.
X-linked recessive disorders are more common in males than in females.
Syllabus 17.4.18: You must be able to use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios.
Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale
Examples include height, mass, finger length etc. where there can be many ‘inbetween’ groups
Discontinuous Variation is when there are distinct differences for a characteristic
For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not - there are no ‘inbetweens’
Inheritance chapter 17 bio
Chromosomes, Genes and Proteins
Chromosomes: made of DNA, which contains genetic information in the form of genes
Gene: a length of DNA that codes for a protein
Allele: an alternative form of a gene
Inheritance of sex in humans is used with X and Y chromosomes.
Haploid nucleus: a nucleus containing a single set of unpaired chromosomes (e.g., sperm and egg)
Diploid nucleus: a nucleus containing two sets of chromosomes (e.g., in body cells)
The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.
Different sequences of amino acids give different shapes to protein molecules.
DNA and Protein Synthesis
DNA: controls cell function by controlling the production of proteins, including enzymes, membrane carriers, and receptors for neurotransmitters
DNA has 2 long strands and 4 nucleotides, AT and CG
mRNA has AU and CG bases
Overall, protein synthesis occurs outside of the nucleus in the cytoplasm
Protein synthesis has two stages:
Transcription (rewriting the base code of DNA into bases of mRNA)
Translation (using mRNA base sequence to build amino acids into a sequence in a protein)
How proteins are made:
the gene coding for the protein remains in the nucleus
messenger RNA (mRNA) is a copy of a gene
mRNA molecules are made in the nucleus and move to the cytoplasm
the mRNA passes through ribosomes
the ribosome assembles amino acids into protein molecules
the sequence determines the specific order of amino acids of bases in the mRNA
All body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs
Cell Division: Mitosis
Mitosis: The nuclear division gives rise to genetically identical cells
Mitosis is needed for:
Growth: in animals, each tissue provides its own new cells when needed.
Repair damaged tissues: for example, when you cut your skin, mitosis provides new cells to cover up cuts.
Replacement of worn-out cells
Asexual reproduction: in plants
The exact replication of chromosomes occurs before mitosis
During mitosis, the copies of chromosomes separate, maintaining the chromosome number in each daughter cell
Stem Cells: unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions
Cell Division: Meiosis
Meiosis: Reduction division in which the chromosome number is halved from diploid to haploid
Meiosis is involved in the production of gametes.
Meiosis results in genetic variation, so the cells produced are not all genetically identical.
Gametes such as sperm and ovum are produced via meiosis.
Monohybrid Inheritance
Inheritance: The transmission of genetic information from generation to generation.
Here are some common terminologies you should know to understand monohybrid inheritance.
Terminologies
Genotype: the genetic makeup of an organism in terms of the alleles present (e.g. Tt or GG)
Phenotype: the observable features of an organism (e.g. tall plant or green seed)
genotype + environment + random variation → phenotype
Homozygous: having two identical alleles of a particular gene (e.g. TT or gg). Two identical homozygous individuals that breed together will be pure-breeding
Heterozygous: having two different alleles of a particular gene (e.g. Tt or Gg), not pure-breeding
Dominant: an allele that is expressed if it is present (e.g. T or G)
Recessive: an allele that is only expressed when there is no dominant allele of the gene present (e.g. t or g)
Pedigree Diagram
Pedigree diagrams trace the inheritance pattern of a specific characteristic (usually a disease) through generations of a family.
Pure Breeding: The individual is homozygous for that characteristic
This can determine the probability that someone in the family will inherit the genetic disorder.
Genetic Diagrams
Monohybrid Inheritance can be determined using a genetic diagram known as a Punnett square.
A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring.
The dominant allele is shown using a capital letter, and the recessive allele uses the same letter but lowercase.
If you are asked to use your own letters to represent the alleles in a Punnett square, try to choose a letter that is obviously different as a capital than the lowercase so the examiner is not left doubt as to which is dominant and which is recessive.
1:1 Monohybrid Crosses
In this cross, there is a 1:1 ratio of boy to girl, meaning a 50% chance of the offspring being a boy and a 50% chance of the offspring being a girl.
3:1 Monohybrid Crosses
There is more variation in this cross, with a 3:1 ratio of brown eyes: blue eyes, meaning each offspring has a 75% chance of having brown eyes and a 25% chance of having blue eyes
Modification of Mendelian Genetics
Codominance
Codominance: a condition where two alleles of a gene are equally dominant (50%)
Both alleles are equally expressed in the phenotype of the heterozygote.
For example, if the parent phenotype is red and white, a co-dominant will have both red and white colour in the offspring.
ABO Blood Group (Codominance and Multiple Allele)
Inheritance of blood group is an example of codominance
There are three alleles for the blood group given by the symbols IA, IB and IO.
IA and IB are co-dominant giving blood group AB or IAIB, and both dominant to IO.
Sex-Linked Characteristics
Sex-linked characteristic: a characteristic in which the gene responsible is located on a sex chromosome, making it more common in one sex than in the other.
Generally, in the IGCSE syllabus, the most common is X-linked recessive trait.
X-linked recessive disorders are more common in males than in females.
Syllabus 17.4.18: You must be able to use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios.
Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale
Examples include height, mass, finger length etc. where there can be many ‘inbetween’ groups
Discontinuous Variation is when there are distinct differences for a characteristic
For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not - there are no ‘inbetweens’
