How Genetic Information Is Passed On 3.4 Inheritance How Genetic Information Is Passed On

Understanding Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed Gametes are haploid so contain one allele of each gene Two alleles of each gene separate into different haploid daughter nuclei during meiosis Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects Many genetic diseases in humans are due to recessive alleles of autosomal genes

Understanding Some genetic diseases are sex linked and some are due to dominant or co-dominant alleles The pattern of inheritance is different with sex linked genes due to their location on sex chromosomes Many genetic diseases have been identified in humans but most are very rare Radiation and mutagenic chemicals increase the mutation rate and can cause genetic disease and cancer

Important Terms Inheritance Punnett Square Gamete Pedigree Chart Zygote Homozygous Haploid Heterozygous Diploid Allele Gene Dominant Co-dominant Recessive Autosomal Sex-linked

Gregor Mendel The basic laws of heredity were first formed during the mid-1800’s by an Austrian botanist monk named Gregor Mendel. Because his work laid the foundation to the study of heredity, Mendel is referred to as “The Father of Genetics.”

Mendel’ Pea Plants Mendel based his laws on his studies of garden pea plants. Mendel was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly, and have many visible traits such as: Pod color Seed Color Plant Height Green Yellow Green Yellow Seed Shape Short Pod Shape Tall Wrinkled Round Smooth Pinched

Mendel’s Experiments Mendel noticed that some plants always produced offspring that had a form of a trait exactly like the parent plant. He called these plants “purebred” plants. For instance, purebred short plants always produced short offspring and purebred tall plants always produced tall offspring. X Short Offspring Purebred Short Parents X Purebred Tall Parents Tall Offspring

Mendel’s First Experiment Mendel crossed purebred plants with opposite forms of a trait. He called these plants the parental generation , or P generation. For instance, purebred tall plants were crossed with purebred short plants. X Parent Short P generation Parent Tall P generation Offspring Tall F1 generation Mendel observed that all of the offspring grew to be tall plants. None resembled the short short parent. He called this generation of offspring the first filial , or F1 generation, (The word filial means “son” in Latin.)

Mendel’s Second Experiment Mendel then crossed two of the offspring tall plants produced from his first experiment. Parent Plants Offspring X Tall F1 generation 3⁄4 Tall & 1⁄4 Short F2 generation Mendel called this second generation of plants the second filial, F2, generation. To his surprise, Mendel observed that this generation had a mix of tall and short plants. This occurred even though none of the F1 parents were short.

Mendel’s Law of Segregation Mendel’s first law, the Law of Segregation, has three parts. From his experiments, Mendel concluded that: 1. Plant traits are handed down through “hereditary factors” in the sperm and egg. 2. Because offspring obtain hereditary factors from both parents, each plant must contain two factors for every trait. 3. The factors in a pair segregate (separate) during the formation of sex cells, and each sperm or egg receives only one member of the pair.

Dominant and Recessive Genes Mendel went on to reason that one factor (gene) in a pair may mask, or hide, the other factor. For instance, in his first experiment, when he crossed a purebred tall plant with a purebred short plant, all offspring were tall. Although the F1 offspring all had both tall and short factors, they only displayed the tall factor. He concluded that the tallness factor masked the shortness factor. Today, scientists refer to the “factors” that control traits as genes. The different forms of a gene are called alleles. Alleles that mask or hide other alleles, such as the “tall” allele, are said to be dominant. A recessive allele, such as the short allele, is masked, or covered up, whenever the dominant allele is present.

Homozygous Genes What Mendel refered to as a “purebred” plant we now know this to mean that the plant has two identical genes for a particular trait. For instance, a purebred tall plant has two tall genes and a purebred short plant has two short genes. The modern scientific term for “purebred” is homozygous. short-short short-short short-short X Short Offspring Short Parents According to Mendel’s Law of Segregation, each parent donates one height gene to the offspring. Since each parent had only short genes to donate, all offspring will also have two short genes (homozygous) and will therefore be short.

Hybrid Alleles In Mendel’s first experiment, F1 offspring plants received one tall gene and one short gene from the parent plants. Therefore, all offspring contained both alleles, a short allele and a tall allele. When both alleles for a trait are present, the plant is said to be a hybrid for that trait. Today, we call hybrid alleles heterozygous. tall-tall short-tall short-tall short-short X Parent Short P generation Parent Tall P generation Offspring Tall F1 generation Although the offspring have both a tall and a short allele, only the tall allele is expressed and is therefore dominant over short.

Dominant Alleles Mendel observed a variety of dominant alleles in pea plants other than the tall allele. For instance, hybrid plants for seed color always have yellow seeds. Green & Yellow Allele Yellow Seed However, a plant that is a hybrid for pod color always displays the green allele. Green Pod Green & Yellow Allele In addition, round seeds are dominant over wrinkled seeds, and smooth pods are dominant over wrinkled pods.

Law of Independent Assortment Mendel’s second law, the Law of Independent Assortment, states that each pair of genes separate independently of each other in the production of sex cells. For instance, consider an example of the following gene pairs: According to Mendels’ Law of Independent Assortment, the gene pairs will separate during the formation of egg or sperm cells. The plant will donate one allele from each pair. The plant will donate either a yellow or green seed allele, either a yellow or green pod allele, and a wrinkled or round seed allele. It will always donate a wrinkled pod shape. The donation of one allele from each pair is independent of any other pair. For example, if the plant donates the yellow seed allele it does not mean that it will also donate the yellow pod allele.

Genetic Diseases Over 4,000 single genetic diseases have been identified These include Cystic Fibrosis, Sickle Cell Anemia, Hemophilia and Tay Sachs Disease

Genetic Diseases Most of these diseases are very rare because they are recessive genes That means that someone cannot get the disease unless they have two copies of the gene for the disease Someone with only a single copy of the gene will not have the disease but they are a carrier and may give it to their children In order to pass down the disease, each parent must have at least one copy of the faulty gene

Calculating the Chances Question: answer A healthy man whose father had a recessive genetic condition known as Marfan Syndrome marries a woman who is a carrier. What are the chances that their child will have the condition If the man’s father had the condition then the man must have one allele for it. If his wife is a carrier, then she must also have one allele

Calculating the Odds So, if M is the dominant (normal gene) and m is the recessive (disorder) gene, then both parents must be heterozygous or Mm. This means their gametes can be either M or m. We can make a Punnett Square to find the possible genotypes Healthy/non-carrier 25% Healthy/carrier 50% Marfan syndrome 25%   M m MM healthy Mm healthy carrier mm Marfan syndrome

Sex Linked Traits Some genes are located on the sex chromosomes. Since they are not homologous in men, they only have one allele for them. Any diseases that are caused by recessive alleles on the X sex chromosome are more likely to be present in men

Sex Linked Genetic Disease Example – Hemophilia (The Royal Disease) Caused by a recessive gene on the X chromosome, members of several Royal Families in Europe in the 19th and 20th centuries A chart that shows the incidence of a genetic disorder through several generations is called a Pedigree chart This pedigree chart on the following slide shows how hemophilia affected British Royalty. What trend do you notice?

Hemophilia Pedigree Chart

Dominant Genetic Disorders A small number of genetic disorders are caused by dominant genes. In such cases, a parent cannot be a carrier. They either have the disease or they don’t One example is Huntington’s disease. This is a progressive neurological disease that starts to affect someone between the ages of 30 and 50. If one parent has the disease, there is a 50% chance that their child will have it. Question: What are the odds of a child having it if both parents have the disease?

Dominant Diseases Answer: 75% (assuming that each parent had 1 allele)

Co-dominant Genes Some genes are neither dominant or recessive. In such cases both alleles are expressed separately. These genes are said to be co-dominant

Example of Co-dominance Blood Groups Blood groups are determined based on the type of glycoprotein that is produced on the membranes of red blood cells. You receive an allele from each parent. The allele can either be A (adds acetylgalactosamine to the glycoprotein), B (adds galactose to the glycoprotein), or O (adds nothing)

Blood Groups Possible Combinations: Someone with type A blood Genotype Blood Type AA A AO BB B BO AB OO O Possible Combinations: Someone with type A blood will produce antibodies for glycoprotein B. So a transfusion of type B blood would likely kill that person If someone receives blood type O, antibodies will not attack the glycoprotein because there are no attachments to attack. That’s why type O blood can be given to anyone.

What Causes Mutations? Mutations are random changes in the base sequence of DNA. While they are random, the incidence of mutations can be increased by exposure to radiation and mutagenic chemicals.

Assignment 1) Page 183 “Deducing Genotypes from Pedigree Charts” 1-4 2) Page 186 “Data Based Questions: The Aftermath of Chernobyl” 1-4