A trait is a particular characteristic or feature of an organism. Eg. Blonde hair. We inherit many physical features or traits and can also inherit diseases or defects. These traits or characteristics are coded by particular genes located on particular and specific chromosomes. TRAITS & INHERITANCE
Chromosome images are organised by international convention, to assist in analysis. Homologous chromosomes are arranged in pairs according to their relative size, position of the centromere and banding patterns. Autosomes are identified by numbers in order of decreasing size. The pair of chromosomes that differ between the sexes is designated as the sex chromosomes. THE KARYOTYPE
Autosomes Autosomes are the matched pairs of chromosomes that are present in males and females. The number of genes carried on chromosomes vary. Genes carried on the same chromosome are said to be linked.
Genes locate don the X chromosome are said to be X- linked. Females have two alleles of a particular gene whereas males will only have one (as they only have one X chromosome). This accounts for why many X- linked diseases show up more often in males than females. Genes on the Y are Y-linked. In mammals it is the male that determines the sex of the offspring. HUMAN SEX CHROMOSOMES The Y chromosome is a degenerating X chromosome.
Alleles Genes occupying the same position (locus) on homologous chromosomes are called alleles. Alleles are versions of the same gene that code for a variant of the same polypeptide. Any one individual can only have a maximum of two alleles for a given gene. There may be more than two alleles in a population, e.g blood groups A, B, O. Gene A Genes that occupy the same locus code for the same trait. Gene B Gene C Paternal chromosome Maternal chromosome Pod color in peas is a trait controlled by a single gene. The allele for green pods is dominant over the allele for yellow.
When both chromosomes have identical copies of the dominant allele for a gene, the organism is said to be homozygous dominant for that gene. Alleles These two different versions of gene A create a condition known as heterozygous. Only the dominant allele (A) will be expressed. When both chromosomes have identical copies of the recessive allele for a gene, the organism is said to be homozygous recessive for that gene. Genes occupying the same locus or position on a chromosome code for the same trait and are said to be alleles. Paternal chromosome that originated from the sperm of this person's father. Maternal chromosome that originated from the egg of this person's mother.
A mutation is a heritable change in genetic material. Mutations are sources of new alleles. Mutations in genes can involve changes in small parts of the DNA sequence. MUTATION
A change in a gene which is unpredictable is called a gene mutation. Some mutations occur in normal body cells or somatic cells. They often have no effect on the individual. But sometimes they produce a lump of cells called a tumour. If a tumour spreads it is known as malignant and the diseases caused by such tumours are cancer. Mutations
Mutations in somatic cells cannot be passed on to offspring by sexual reproduction. If the mutation is in the testes or ovaries, known as germ line cells then the mutated gene can be passed on.
There are two ways in which DNA can become mutated: Mutations can be inherited. Parent to child Mutations can be acquired. Environmental damage Mistakes when DNA is copied Mutations can occur spontaneously WHAT CAUSES MUTATIONS?
Point mutations occur due to the alteration of a single base in DNA. The following are point mutations. Silent mutation results in a change in the DNA sequence which does not result in an amino acid change. This is due to the fact that some amino acids are coded by multiple triplet codes. Missense mutations occur when a single substitution that results in the replacement of the amino acid. The consequences of this type of mutation depend on the protein involved. Mutations that result in the generation of a stop codon are termed nonsense mutations. These mutation result in the stop codon (UAU or UAA). These can have severe consequences. TYPES OF MUTATIONS
Frameshift mutations: If a base pair is added to or deleted from the DNA, the wrong amino acids are incorporated for the rest of the sequence. This is called a frameshift mutation.
Genotype is the actual double set of genes (alleles) possessed by the individual. One from each of the homologous pairs in a diploid cell. The number of possible genotypes depends on the number of allelic forms of the gene. Think about: Hair colour in humans Eye colour in humans Feather colour in budgies All are very varied so must have many alleles in the population. GENOTYPE
Phenotype is the physical expression of the inherited trait. The expression may be a physical, biochemical or physiological trait. A dominant phenotype is a trait which requires only a single copy of the responsible allele for its phenotypic expression. It will expressed in heterozygous individuals. A dominant phenotype is represent by a capital letter. A recessive phenotype is a trait which expressed only in homozygous individuals. A lowercase letter is used to represent recessive phenotypes. Genotypes: BB = homozygous ( BB and Bb will have the same Bb = heterozygous phenotype. ) bb = homozygous PHENOTYPE
20 MENDEL’S LAWS 1. LAW OF DOMINANCE 2. LAW OF SEGREGATION 3. LAW OF INDEPENDENT ASSORTMENT
21 LAW OF DOMINANCE In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds)
22 PUNNETT SQUARE Used to help solve genetics problems
DOMINANCE AND RECESSIVE A trait is dominant when the heterozygote and one homozygote have the same phenotype. A trait is recessive when the phenotype is observed only in one homozygote
25 LAW OF SEGREGATION During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
1. Every trait (like flower color, or seed shape, or seed color) is controlled by two "heritable factors". [We know now that these are genes - we each have two copies of every gene]. 2. If the two alleles differ, one is dominant (will be observed in the organisms appearance or physiology) and one is recessive (cannot be observed unless the individual has two copies of the recessive allele). Dominant traits mask the appearance of recessive traits. 3. Alleles are randomly donated from parents to offspring - the factors (alleles) separate when the gametes are formed by meiosis, allowing all possible combinations of factors to occur in the gametes.
DOING A GENETIC CROSS (MONOHYBRID = 1 GENE): Geneticists use letters be used to represent alleles. A capital letter = Dominant trait, a lowercase letter = a recessive trait. The same letter is used to indicate both alleles. Examples = Flower color: P= purple, p= white = Seed color: Y= yellow, y = green = Seed shape: W = wrinkled, w = round In humans... = Widow's peak: W = widow's peak, w = continuous hairline = Freckles: F = Freckles, f = no freckles (which are you?) = Earlobes: E = unattached, e = attached (which are you?) = Cystic fibrosis C = no CF, c = cystic fibrosis
E-ZY STEPS FOR DOING GENETICS PROBLEMS: 1. Indicate the genotype of the parents using letters 2. Determine what the possible gametes are 3. Determine the genotype and phenotype of the children after reproduction. To consider every type of offspring possible, use a Punnett Square in which all possible types of sperm are lined up vertically and all types of eggs are lined up horizontally: 4. Fill in the squares by "multiplying" the alleles from male & female.
GENETICS PROBLEM 1: (a) A man with a widow's peak (WW) marries a woman with a continuous hairline (ww). A widow's peak is dominant over a continuous hairline. What kind of hairline will their children have?
GENETICS PROBLEM 1: 1. P1 Widow's peak (WW) x continuous hairline (ww) 2. Gametes: Male: W only, Female: w only 3. Children: (the F1 generation): P1WW x ww Gametes W w F1Ww - Ww Genotype – Ww ( heterozygous) Phenotype- widows peak all children
(B) SUPPOSE ONE OF THEIR CHILDREN (WW) MARRIES SOMEONE WHO IS ALSO HETEROZYGOUS (WW). WHAT TYPE OF HAIRLINE WILL THEIR CHILDREN HAVE? P1 Ww x Ww Gametes W w male W w female Genotype: Their children have a 25% (1/4) chance of being WW, a 50% (2/4) chance of being Ww, and a 25% (1/4) chance of being ww. (Note that this is a 1:2:1 genotypic ratio IF both parents were heterozyhous to begin with) Phenotype: Their children will have a 3/4 chance of having a widow's peak and a 1/4 chance of having a continuous hairline (3:1 phenotypic ratio) Ww WWWWw w ww
MORE PROBLEMS Genetics problem 2: A man and a woman are heterozygous for freckles. Freckles (F) are dominant over no freckles (f). What are the chances that their children will have freckles? Genetics problem 3: A woman is homozygous dominant for short fingers (SS). She marries a man who is heterozygous for short fingers (Ss). Will any of their children have long fingers (ss)? yes / no
2 GENE INHERITANCE So far we have looked at inheritance of a single gene. This is called monohybrid. Inheritance can involve two genes: for example eye color and hair color. We call this dihybrid.
REVIEW Gametes (sex cells) only receive one allele from the original gene. As you know, 2 alleles control a gene. "Ee" is a gene in the critter opposite. When gametes are produced, the alleles of the gene separate and go into different sex cells; in other words, one letter is packaged in one sex cell and the other letter is packaged in another. Please note that each sex cell contains 50% of the original gene. Ee = gene of the Hybrid parent.
PRINCIPLE OF INDEPENDENT ASSORTMENT Independent assortment occurs because in meiosis the segregation of one pair of homologous chromosomes (and the alleles they carry) does not influence the segregation of other homologous pairs of chromosomes A ratio of 9:3:3:1 will be observed if the following conditions apply: Two genes control two traits There are two alleles for each gene For each trait one phenotype is dominant Both genes are on autosomes The two genes assort independently
THE LAW OF INDEPENDENT ASSORTMENT It appears that the inheritance of seed shape has no influence over the inheritance of seed colour The two characters are inherited INDEPENDENTLY The pairs of alleles that control these two characters assort themselves independently The pairs of chromosomes could orientate in different ways at Anaphase 1
RULE OF INDEPENDENT ASSORTMENT All this means is that the random selection of one trait will not determine the random selection of another. (This of course assumes that the genes are on separate chromosomes.) In other words, the genes for your eyes are transmitted independently of the genes for your height. Your kids could be tall with brown eyes. Or, your kids could be tall with blue eyes. The traits, or the alleles, assort independently of one another.
DIHYBRID CROSS Mendel discovered independent assortment from following two traits simultaneously. If the traits are caused by genes on different chromosomes, they will be inherited independently of one another. If the first cross is between individuals homozygous for both traits the resulting offspring are heterozygous for both traits and are called dihybrids. A cross between these offspring is a dihybrid cross. So what is the chance that we can inherit two genes from a parent which are located on a different chromosomes together. We work this out in the same way as monohybrid crosses by using a punnett square.
ALLELIC NOTATIONS First we need to assign allelic notations to the genotypes. Or in plain terms ‘symbols’. Gene 1 - B = Brown eyes b= blue eyes Gene 2 – H = Black hair h = Blonde hair To work out if the genes are on different chromosomes or if they are on the same chromosome you need to perform a cross.
THE CROSS The parental generation will be homozygous x homozygous dominant recessive. BBHH x bbhh Brown eyes black hair x blue eyes blonde hair This is the Parental generation. The first step is to work out the gametes that the parents pass to F1 generation
GAMETES BBHH The gametes for this parent are the different combinations of B and H you can have. No matter how you arrange these you will always get the same BH. Remember that Gametes have half the chromosome number (haploid) that the parents will have. So they will pass on half the letters. What will the gametes be for bbhh?
CROSS ONE To work out the F1 generation we cross the gametes from each parent. BH x bh So all the F1 generation will result in the same genotype BbHh Brown eyes and Black Hair BH bhBbHh bhBbHh
CROSS TWO We then take F1 generation and self cross them. To do this we need to again work out the possible gametes that they have. In this case it is every combination of both genes that includes at least one from each. So you must have either a B or b and a H or h. It also reduces the chromosome number by half. The gametes will be the same for both BH, bH, Bh, bh. You now put these into a punnett square to get the F2 generation.
DIHYBRID CROSS Dihybrid Cross. AA or Aa = purple; aa = white BB or Bb = tall; bb = short
Incomplete dominance With incomplete dominance, the phenotype is only partially expressed in the heterozygote. The phenotype of the heterozygote is different from either homozygote. A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype. For example, in cross- pollination experiments between red and white snapdragon plants the resulting offspring are pink.
Flower colour in snapdragons is an example of incomplete dominance. So what is wrong with how the genotypes have been expressed in this diagram?
53 INCOMPLETE DOMINANCE have an appearance somewhat the of the two parental varieties. F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. x white (rr) red (RR) x white (rr) RR = red flower rr = white flower r r RR
54 INCOMPLETE DOMINANCERrRrRrRr r rRR All Rr = pink (heterozygous pink) produces the F 1 generation
CO-DOMINANCE Both the alleles can be expressed Eg. Red cows crossed with white will generate Roan cows. Might seem to support blending theory.
Codominance Codominance occurs when the full phenotypic expression of both alleles is observed in the heterozygote Eg Blood types: where there are three allelic forms at the same locus and individuals can have A, B, AB or O phenotypes.
57 Codominance Two alleles are expressed in heterozygous individuals. Two alleles are expressed in heterozygous individuals. Example: blood type Example: blood type 1.type A= I A I A or I A i 1.type A= I A I A or I A i 2.type B= I B I B or I B i 2.type B= I B I B or I B i 3.type AB= I A I B 3.type AB= I A I B 4.type O= ii 4.type O= ii
In humans, there are four blood types: A,B,AB and O Blood type is controlled by three alleles: A,B,O O is recessive, two O alleles must be present for the person to have type O blood A and B are Codominant. If a person receives an A allele and B allele, their blood type is AB type Crosses involving blood type often use an ‘I ‘ to denote the alleles
59 Summary of Dominance Relationships X Dominance complete X incomplete X codominance
Environment affects some phenotypes While the phenotype is subject to inputs from genes (genotype), it may also be affected by the environment. Thus, if an individual with a given genotype develops in one environment, its phenotype may be different than if it had developed in some other environment. ‘Identical’ but different twins: For any given trait, differences in phenotype between identical twins cannot be due to differences in genotype and must be attributed to differences in the environment, for example the effects of different diets, different exercise patterns and different climates.