If you want to change gender dropdown to radio button in magento this tutorial will help you. By default magento having a gender dropdown on registration page. On registration form magento uses widgets, you can see following lines on template register.phtml.
<?php $_gender = $this->getLayout()->createBlock('customer/widget_gender') ?>
<?php if ($_gender->isEnabled()): ?>
<li><?php echo $_gender->setGender($this->getFormData()->getGender())->toHtml() ?></li>
<?php endif ?>
So you can found gender widget template file on template\customer\widget folder. Now to change gender dropdown to radio button replace below code into the file.
<div class="input-box">
<?php $options = Mage::getResourceSingleton('customer/customer')->getAttribute('gender')->getSource()->getAllOptions();
$value = $this->getGender();
$_last = count($options);
$i = 0;
?>
<?php foreach ($options as $option):?>
<?php if($option['value'] != ''): ?>
<input class="radio<?php if ($this->isRequired() && $i == $_last - 1):?> validate-one-required<?php endif; ?>" type="radio" title="<?php echo $option['label'] ?>" value="<?php echo $option['value'] ?>" name="<?php echo $this->getFieldName('gender')?>" <?php if ($option['value'] == $value) echo ' checked="checked"' ?> />
<?php echo $option['label'] ?>
<?php endif; ?>
<?php $i++; ?>
<?php endforeach;?>
</div>
Hope this tutorial will help you to change gender dropdown to radio button on registration in magento.